The reality is that I wouldn't do that, since there are a lot of things that go into an engine that I wouldn't want to have to deal with.

Instead, I think I'll take the code from Balloon Ghost and Robot Ops and extend it for the sequel to Bad Blaster. I'll focus on 2 things specifically:

Unreal Engine uses some fancy magic to blend two animations together. This is used to make a character smoothly transition from idling to walking, walking to running, and from anything to jumping. I wouldn't build out a fancy GUI for the transitions, though. I would just write the code to blend two animations together for a duration of time.

Blending the animations together would look really good on a more detailed flat design style.

I'm still using Tiled to make levels. I have no reason for wanting to replace it, as it's a great level editor. However, I sometimes want to see and play my level instantly, and I want to swap things out faster. I'm thinking of doing something like Mario Maker's level editor to make the levels.

After gaining a ton of experience writing code, the prospect of building such a thing for a simple 2D game doesn't daunt me. I'm much more confident and capable than I was when I first started making games, and so having a proper level editor would only end up making the final game much more enjoyable for the player in the end.

Of course, both of these things depend on whether I will make Bad Blaster 2 or not. As of right now, that's still only a possibility, not a certainty.