Before start coding you should be having a plan, some vision of what you want to get. And you shouldn't invent something new when almost everything is done - that would be frustrating.
Because changing even smaller parts, or adding something may lead to reconsidering many things. I'm saying about planning things before implementing them.
Do you, Daiky, want to face situation when you'll need to redo many things? I bet you don't, me either. Though, I can presume you probably don't really care about where this project will go after you get battlescape part working "similar to original". Hell, probably nowhere.
This topic is about essential questions, and with _real_ answers (not wishes) we can say what this project will look like. Or what we need to aim to.
I'm not going to some "offtopic". The topic is about "long term data structures". Not current data structures (borrowed from
https://ufopaedia.org and original game), which can hardly be extended, but structures which will be topical after 1.0.
For instance, even getting 2x sprites and resolution will require for new formats, and for rewriting alot of code, just because with current "shading" sprite engine it will be pretty unplayable at 640x480.
Why am I seeing some sort of a dead end after 1.0?