I think this one is pretty clear and understandable honestly.
Although goblin slayer will be extremely weak in Orairo. I was thinking more of a "non dungeon diving" adventurer. I've heard that goblins also existed outside of dungeons, like in the Goblin Slayer anime.
Also, it would be interesting to see him meet various of characters. The fact that he only hunts goblins is also pretty unique and realistic for Danmachi.
Maybe make him unable to recieve falna? And because of that fact either give him a tragic backstory (Maybe his generation was cursed by the gods so that he couldn't recieve falna and because of that his house getting raided by goblins and him watching whatever transpired in Goblin Slayer.) or some kind of motive(dunno).
I have no idea if this Idea has been done before, but I wouldn't be surprised if it has. After all, Goblin Slayer is really cool.
AN: Sorry for being gone for a while, but I still can't promise any consistent upload schedule.