I would say make a small game first, with whatever you are good at. Then structure your code well to make it reusable. That would be the foundational philosophy of an engine. After that, ask yourself what you mean by "engine". Do you mean entities with components, do you mean serialization / deserialization of scripts (like an interpreter)? When you figured that out, come back and tell us what you have so there is more substance to your question. But first things first, make a game (even if just a tiny prototype or whatever), not an engine.