My thoughts too Sukhe. Unfortunately I've already started some BoB NW Frontier shenanigans in 28mm so that would be 2 lots of terrain to make ..........
Still 15mm Sudan looks a possibility.
It's the same reason I've stuck with 15mm, though I have been very tempted to paint up some 28mm Zulus and British using Empress Miniatures to use with the Battlegames supplement.
Timmo - probably PP for the most part. Martin Goddards recent ranges are first rate IMHO & he's now done Highlanders. I'd fill in the gaps with OG & Minifgs as necessary & for some variety but there may still be some holes.
I try to keep the terrain reasonably multi scale where practical so it's only a matter of changing buildings really. It's time (& currently money) that prevents any real progress.