>>715898856 (OP)Luigi's mansion but Amy. Give her her playstyle from adventure 1 and have her wander around a set of horror themed open zone areas leading to a few levels each. A spooky forest, an old decrepit mansion, then a secret underground eggman base thats torn apart and malfunctioning rather than actively hostile like his stuff normally is. Have the enemies be weird goopy ghost like creatures that poses busted mechs or trees or even animals and turn them into scary aggressive versions of themselves and attack you.
The plot is that Sonic went off to beat Eggman again in another adventure recently, but he didnt come back. And Eggman seems to be both excited that Sonic seems to be gone, but also wary of whatever happened between them. Amy gos in to try and find Sonic as she is want to do and ends up having to brave all of these scary environs on her own. Maybe running into some of the other characters making their own way around along the way.
Eventually its revealed that the adventure sonic went on was much like previous ones: Eggman tried to harness the power of some evil entity, Sonic went to stop him, but this time Eggman actually listened and realized that what he was messing with would likely destroy the world/take over his empire/etc. and should probably be left sealed... right at the last second. As a result the entity was partially unsealed, managed to capture Sonic, Eggman paniced and initated a self destruct of his base and high tailed it out of there. All the enemies Amy has been facing have been minifestations of the entities possession trying to guide her into unsealing it fully using Sonic as bait. The final boss is either a fight against Sonic's possessed body as she tries to take him back from the entity or a fight against some powerful avatar while Sonic is trapped in a magical cage or something.
True final boss can be a super sonic style boss fight where amy gets to tag team with sonic to beat it once and for all.