Well, I can tell you one thing: the gameplay is too slow moving. At that speed there, I'd press the jump button, and fall asleep before the character landed.

Level-creators are practically just "high-level" game engines that try to maximize versatility without expending simplicity. Much of what you'd be doing is just mapping unreal-engine features to a more user-friendly GUI in the view (this is harder to do than you'd think.) Since you said you want the person creating the game to import their own assets, it seems to me as if you don't need to worry about producing music, art, etc. Just a pretty and easy to use GUI including all of the unreal features that most people would be interested in.

This would be neat if you pulled it off, but it would be quite hard to do. Most level-creators have a specific physics and asset template (Super Mario Maker uses various Mario games as its template, for example.) This is what makes them so great. It feels like you can make anything, but you are indeed being limited. Any more freedom gets to the point where using a real game-engine is just better.

The end result might turn out to be like older versions of RPGmaker in which every game produced by it felt generic.

In fact, I would look into how RPGmaker was made, and what features were added with time to make it better, if I were you. That is essentially what you are trying to make, but for the platformer genre. Of course you'd have to learn a decent bit of programming along the way (game-programming is pretty easy in my opinion.)

Unreal Engine is a bold choice, I think. Unity is a very easy engine, and having tried Unreal Engine after Unity, it felt really hard and awkward. I'm sure it's very efficient and not that difficult once you get used to it, but it was pretty hard.

Yeah, I suppose this isn't entirely on topic. :P Just thought I'd share my experiences so far.