Pokémon Snap is literally the only game I can think of right now that desperately needs a sequel. Or hell, to be turned into a full-fledged series. :( I'd snatch up those games faster than they could announce them, omg.
Otherwise, I can't think of any games I've played that I want a sequel for. Either because they were so good they already got sequels or because they were perfect the way they were. TWEWY's an example of that--I loved the game and my first thought leaps to "omg want another game" but... it was so perfect by itself. It stands alone so well and I think the experience would be tainted if it turned into a series. I'd be too scared it wouldn't be done right, since it was already a very hard game to balance as far as appeal goes. They did everything right with it, though, and I'm afraid any tweak to the formula, which you need in a sequel, would ruin that balance. D: