In regards to canon, I don't think Ultimate Ben is actually an Ultimate. I think it's more like the Biomnitrix in Omniverse - Ben 10000 altered the Ultimatrix to use abilities in human form.
This is more my ideas for how an actual Ultimate Human might work. My idea is to double down on what humans are best at in real life - long distance running.
In Omniverse, the Ultimates tend to be larger than the base form. So where normal humans get to 6 feet tall on average, an Ultimate Human would be 8 or 9 feet. (I know some people do get to 8 feet in real life, but that comes with health issues, such as their legs not being able to deal with the weight. Since this is artificial evolution, an Ultimate Human would be fine.) They'd have tougher bones and stronger muscles in general. They can pick up a car pretty easily, but would struggle with a tank. (Most strength-based aliens can lift a tank easily in base form.). Note that the transformation would still be closer to a runner's build than a bodybuilder's.
Longer feet for more speed. Modern human feet are designed for force over speed. Capable of running at 95 mph.
Barrel Chest. A larger chest (proportionally as well as larger in general) means larger lungs and heart. Combined with the blood cells being MUCH better at holding oxygen, means this Ultimate Human can run nonstop for two weeks if need be. They can also hold their breath for longer. Helpful since, due to the different proportions, Ultimate Humans aren't as good at swimming anymore.
Ultimates generally work on a 'Lose a power, Gain a power' rule. Ultimate Humans have already gotten worse at swimming, so what new thing are they getting?
Scalding. Hot. Breath.
This is basically going off of the fact that one of the reasons Humans are so good at running is because our body's natural cooling - aka sweating- keeps us from overheating. Having a larger body that moves far quicker than normal means much more heat is produced. While this might be good for winter (which is why Ultimate Humans aren't super hairy), they still needed to evolve a new way to cool off.
Ultimate Humans can redirect the heat in their bodies to their large lungs, and expel out a stream of hot air. The more they've run, the hotter their breath is. This also means that Ultimate Humans are far better at handling internal temperature changes than us regular humans. Change us a few degrees, and we're in bed for a week; meanwhile, these guys are changing their internal temperature at will by several dozen degrees. Getting hit by these sudden streams of heat can cause severe burns. (The Ultimates are living weapons, after all)
Enhanced IQ. Faster learning, better memory. All that jazz. The interesting part is that the brain is the same size. In real life, evolution is now making our brains smaller - we're not getting dumber, our brains are just getting more efficient at what they already do. The idea here is that going from base to Ultimate would have this happen in a cycle -bigger smart brain, smaller efficient brain, repeat- until the Ultimatrix simulation ends.
Bigger eyes, proportionally. Able to see far more detail from further away. Great night vision - they dilate more than regular human eyes. "Sees faster" - in other words, the brain processes visual information way better. Great when running at high speed.
I also headcanon that every Ultimate triggers an Uncanny Valley effect in the normal population of the species - it's them, but also not.
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Bonus: Small Headcanons I came up with when bored.
Plural of To'kustar is To'kustrella
Anur Vladias and Anur Transyl were originally called Anur Bromstök and Anur Wollstonia, but were renamed after the Vladat Royal Family that ruled them.