Meow button would be nice; would have a lot of potential. It could be a little easter egg for those pressing all the buttons or it could be an essential game mechanic, or anything in-between. 'Course it's entirely possible that it doesn't fit with the game but it'd have a lot of potential for parts of the game, unfinished or finished.
Random ideas (just suggestions):
-Could be used to alert guards to move them to your position. May be useful for speedrunning if you want to disrupt pathing.
-Could be used in conjunction with powerups/upgrades to add more movement options to the game. For example, a meow that hurls you backwards, even while in the air.
-Could be used as an emote (like crouch spamming in MC) if using multiplayer. If you can't press crouch a whole bunch of times, you could press the meow button a whole bunch and have a similar effect. This assumes multiplayer is planned, which might not be.
-Another multiplayer feature; could be used to alert other players to your position. This assumes multiplayer is planned, which might not be.
-Could be used to echo-locate (suggested by a friend) so you can see in dark areas.
-Could be used to attack guards or activate things.
-Could have different kinds of meows for multiplayer (assuming multiplayer is planned)?
These are just ideas; They don't need to be implemented but they're there if need be. Also, I know like 1/2 of the suggestions assume multiplayer is possible and that's something that may not happen, even if it's just "I see your character where you are and that's it." Again, great game, and great idea!