I mean, appart from being the next Avatar we know absolute jack about the MC of Four Elements Trainer. He doesn't have an identity of his own. And that's a very common trope, in lewd games or in other games.
Like I said before, it's a nice change of pace in my opinion. I don't know just how irrelevant or unintesting that character was in the original material though so I hesitate to say if it was based on Avatar I'd be cool with Playing as Uncle Hiro. He's one of the best characters in that show.
I do agree that it's a premisce that looks like it can only go the direction of playing a villain who's just gonna corrupt and turn all the women around into his personal onaholes or some whores to make money off of or whatever. And that's common too I guess. Plus it's not really something I'm hugely into. So, if it turns out that way that's a black mark against the game for me too.
We know he is a coward,a pervert(complete with his own particular brand of humor reflecting that),rather pessimisitc as well as incredibly uncaring about the state of the world.
Not saying he is especially deep or established but its a far haul from a blank slate self insert(and the later books do give him at last a bit of character development).
I dont remember too much of Naruto post timeskip(the writing went downhill so hard its hard to believe it ever reached any amount of popularity...then again even I say the very early Naruto was actually pretty decent so I guess that suckered everyone in)but from what I do remember the Raikage actually was a rather badass and upstanding character(I think anyway...they all just started to melt together so damn hard)!?
Probably could work as a subversion to the trope(ie getting the girls to fawn over him by pure manlyness instead of the tired corruption route)but alas I have no clue what the makers actual plans are.