Need help with campaign concept.

So in about a year I’ll be dming a full campaign for the first time, I’ve dmed one shots plenty of times and am currently dming what I’ve been referring to as a mini campaign which will only last 4-5 sessions. I have a few plans for the campaign so far, the main villain is going to be a super advanced wizard mimic with an army of sentient mimic’s I’m going to allow the players to play as any of the normal dnd classes along with the Valdas spire of secrets classes. My problem is I feel like I’m missing something, like a gimmick or an alteration to the main dnd system or something. Idk if anyone has any ideas that might work with what Im doing or honestly any ideas in general I’d love to hear them, even if they’re not relevant I just need something to get ideas from at the very least.