I think everyone agrees that it's hard to imagine what could be in the basement that can live up to all the hype that's been generated.

I want to say right off the bat that this is purely my own opinion, but what's in the basement hasn't me hyped all that much. Or rather, I don't see myself being disappointed by whatever is in there.

I'm pretty sure that, with this being SnK and all, everyone will finally get to the basement to get their "loot", only to find something that they don't comprehend (yet), something that only leads to more questions rather than answers.

Honestly, I think it would fit the end of this "arc": everyone pretty much died for the sake of this (side) goal, but for what? What I'm saying is that, at first, it's gonna be exactly like the fight with Annie; lotsa people died, they made a huge gamble, but in the end Annie crystalized herself and they didn't get anything out of her.

I'm thinking something like that will happen (again, AT FIRST), leaving the few that are left in shambles and despair, only for Eren to suddenly "remember" a certain memory or something that will point them towards the right path (outside of the Walls, most likely).

Again, this is but my opinion. Maybe it will be a lot more simpler and it'll be "good stuff happening" right away.

As for the actual contents of the basement... No idea. All I/we know for sure, is that Grisha was probably using Mikasa's blood or something, maybe he wanted to mass-manufacture her blood/genetic code so that everyone would be protected against the memory-wiping from the Coordinate/Reiss.

Maybe that was his goal all along, hence his interest in the Ackermans; if he just managed to understand what makes an Ackerman immune, he would be able to use said immunity+Coordinate without being under the influence from the First King.

I'll also easily settle with a map of some sort (cue next arc, "adventure beyond the Walls).