Thank you all for your replies, advice and suggestions again.
I've sat down, done some thinking, played a game of Five Men in Normandy with a few figures I've got, and I think I've decided on WWII for my next purchase/project.
Now to decide on a scale, however... torn between 28mm, 20mm and 15mm, I think up to a platoon-level game is what I'm after. I've enjoyed painting all 3 of these scales (however, I need to figure out how to make WWII minis not look like drab blobs in smaller scales - even my 1:72 guys are hard to distinguish in somewhat poor lighting, got any tips?).
Maybe 28mm for the individuality and the character of the figures, and the extra detail? But then again, my local store has a lot of 1:72 vehicles and soft plastic figures and stuff, so it's more readily available, and there's some gorgeous metal sculpts like AB for example... and then 15mm figures and vehicles are even cheaper, the terrain too (and kind of adorable in a weird way), and I can play bigger battles on the same area/more realistic ground scale/on a smaller area, and again there's some really nice sculpts in that scale too...
And then even within a specific scale there's the question of theatre (Western Europe, Eastern Europe, Pacific, North Africa/Mediterranean?) and period (early, mid or late war?) and nation as well
Ah well, we'll see. ¯\_(ツ)_/¯