Can I create a separate game engine
using UE4
If you need to ask that question, then the answer is no.
Can a single, incredibly experienced and motivated individual do it?
Highly unlikely; they will know better than to even start.
Cay I create an in-game editor that
allows for creating simple mini games?
Absolutely yes.
UGC can be made. Procedural meshes, modular instancing and creating textures in real time can be done.
edit:
I wanna create an easy simple engine
to create something like specifically
a 3D puzzle creator or something
You’d better off writing tools, utility blueprints, function libraries and plugins.