This is an idea that I got from a dream I had the other day. I will tell you my dream and then tell you the actual novel idea I had for it. Alright my dream went like this:
I woke up to a loud noise and something like dirt falling on my head. I got up and looked around and saw I was in a small hospitable for some reason though I remembered being in my room when I went to sleep. I walked around and saw that, even though I was in a hospitable, everything was in a weird language that I didn't understand, but I could still recognize it was a hospitable or clinic of some sort. The hospitable looked abandoned and old, there were cracks in the walls and ceiling and dust everywhere with trash spread around. Though I recognized that it was a medical facility I couldn't recognize the facility itself or the language the stuff was written in as it didn't appear to be any notable language from Earth but also seemed to be more simple than any language on Earth but also more sophisticated at the same time. After walking around a bit, I decided to head outside and I could hear noises outside that sounded like gunshots for some reason. I walked outside and was greeted with a crazy sight of a fully grown man flying towards the place I just walked and of and exploded against the wall. For some reason, I didn't freak out immediately but got surprised at how strong the seemingly thin walls were for them to not even shake after being hit with that much force, then got scared thinking of the holes in the building and cracks in the walls and ceiling. It finally hit me, after a few minutes, that I just saw someone die by splattering against a wall and freaked out and turned to see what threw him. When I turned I saw a group of people wearing tight fitting, black army fatigue like things shooting at a giant monster. the giant monster was bleeding from a few places but didn't seem too hurt. There were about 8 people shooting at it with seemingly futuristic looking guns, but they definitely appeared to be on the losing end of things. The monster looked unlike anything I've ever seen before; it had one eye, messy brown hair, two horns sticking out of the sides of it's head, with Caucasian skin, with two arms, wielding a club big enough to hit boulders like a baseball, fangs sticking upwards out of his mouth, and wearing a diaper. The people weren't doing much to it and were, after about 10 minutes, completely wiped out. I was silently scared, just staring at what happened when it noticed me. I decided to run when I noticed the environment was unlike anything on Earth; large rocks with vegetation on them floating in the sky, some rocks seemed big enough to be islands and had a waterfall running off the edge, while the place I was at was still at the front of the medical facility but there were rock walls surrounding us with vines stretching down them at some parts, the rock wall was big and bumpy with some parts jutting out and wasn't smooth at all, but none the less the scenery was beautiful. The monster was in a flat area fighting the people, but was now running towards me at a pretty freaky speed with extremely loud footsteps that cause shaking every time his feet landed. We had about 2 football fields length between us, so I turned and ran around the medical facility and saw I cave and ran into it and saw a river in it so I jumped into it, though not before turning around and flipping the monster off, and as I fell I realized I didn't know how to swim and blacked out and woke up falling down a waterfall into a lake. I floated to the shore and , while struggling greatly, doggy paddled to the land and vomited what felt like a swimming pool worth of water and looked around. What I saw was a nature preserve building, or what looked like the buildings I saw when I went to the zoo, and most importantly had people in them. I ran to the building, not screaming, but just waving my arms and they noticed me and responded by aiming guns at me, to which I raised my hands above my hands with a helpless smile. They came to me and checked me for weapons and asked what I was there for in a language I didn't recognize but could understand and I responded with an I don't know which was met with weird looks as they didn't recognize English but understood it as well. I was taken inside where I was asked many questions and also asked some questions myself. I found out that I was on a world which they called El Primer Mundo, which means The First World in Spanish, which I couldn't understand why they new Spanish but not English, but oh well. They explained the situation of the world to me. Apparently, the world was once striving with extremely advanced technology only scene in sci-fi movies, but one day the world changed to what it is now. After everyone got used to the new world, they came. They were aliens, only weirder than you would think. There were four types of aliens: Grunts, Giants, Brains, and Commanders. The grunts were red, lobster/crab like things that had good speed and could slice a human in half easily, the tanks were the ones that I had saw that chased me, they are fast, strong, and stupid but also come in different sizes with the smallest ones being about 6 meters tall. The small ones were smarter, stronger, and faster than normal tanks. The grunts also had many variations but they are mainly foot soldiers, so they don't really matter. The aliens usually sent the biggest tanks when they decided to take over a city. These monsters were 50 meters tall and with their gigantic clubs could take down even the biggest buildings in the world. This might seem contradictory because I just said the smallest ones are the strongest but even they aren't as strong as the 50 meter giants because the giants are slow and dumb and only made to exist for laying siege to cities but the small giants are fast, nimble, and smart and are sent to hunt down strong factions and parties of humans for capture. The brains look like albino gorillas with no hair on their hands, chest, and face and look like smart gorillas like Winston from Overwatch in a way but albino and less hairy. These guys are the same size as gorillas and have about the same strength as gorillas but are extremely smart and also wear glasses and carry a clipboard with a touch front on it, which is weird cause they could just bring a tablet but who am I to judge space monkeys in my own dream. These brains are usually sent to make peace with certain factions and find out the strength of others and make deals with them. Lastly, are the commanders, they call them this because they are the strongest, smartest, fastest, and best of all the aliens. The commanders are about 6 feet tall on average and look like humans in every way except there skin, their skin varies depending on what kind of powers they have. Some have blue, some have red, some have purple, some have grey, it doesn't matter. What does matter is that they have powers, like supernatural abilities. Their are tiers of strength with commanders because some are exponentially stronger than others, but they all have powers of some kind. This made the original people of El Primer Mundo despair until a discovery was made. Once the world changed and many people died, it was discovered that humans have developed a new organ inside them. This new organ was called something I don't remember but it's not important. What the organ does is allow you to evolve and once you reach a certain level you gain a supernatural ability to fight against the alien invaders, but this doesn't change you skin color but rather your hair and eye color. You evolve by pushing yourself to the limits and breaking your limits, whether it be in a fight or in training. After learning this, I sat in silence for a while before looking around and realized none of the people here evolved and smiled bitterly. I went to find something to do and eat when I heard noises outside and went to the window and saw a giant ufo like thing lowering a ramp with blinding white light coming from the inside from the gap in the ship made by the ramp. I saw three people getting off the ufo, one with skin light blue looking like a god of snow and ice, the other with red skin as red as lava and flame looking like he ruled over all flame, but what was surprising were that the two beings that seemed to be crazily strong were following behind the person in the middle like two body guards. The person in the middle had a golden band floating above his head with skin a dark as night, so dark his skin seemed to be a black hole for all light. He seemed to be divine and powerful and was oddly beautiful, with a charisma that only people as strong as him could possess. I was amazed by the beauty and strength of this person when he looked at me and smiled and then I woke up.
It was honestly one of the most interesting dreams I have ever had and I thought if you tweaked with the story a bit you could make a pretty good novel out of it, cause I don't see many apocalyptic novels with some novels having aliens in them but I just don't see that many that are apocalyptic that are about aliens and new worlds and such. I think that if you give the MC some back story and go more into world building and how El Primer Mundo changed to be the way it did, then I think that it would be a pretty interesting story that could eventually evolve into the Main Character making friends and factions to eventually free the world from the aliens and find the reason they are there. This could open up many potential pathways for the story to go such as realizing that commanders are not even that strong and merely the most ordinary of the bunch or introducing new aliens that fight for right of rule over The First World with the MC having to now fight off both of them, or the MC could beat them and steal their technology leading the world to space travel and eventually finding Earth and stuff like that. This could even expand and you could write a sequel to a book about this idea, depending on what path you take with it.