So I tend to build a couple of different armies with the same basing scheme - either ones that can ally or share units.
Have dark age undead and varangur with a tundra/slight snowy vibe; SK/OotGL/FoN with fallen pillars and greenery; KoM/Brothermark/Halflings based around a ruined village style theme; KoM/basilea/FoA with city/temple vibe.
Usually I’ll figure out a basing scheme and build a number of ‘standard’ sized bases (infantry/cav regiments, some troops etc), paint those up and figure out what goes on them afterwards!
The mantic terrain crate range is excellent and some of the GW scenic stuff is nice.
If you have existing terrain and a table which matches, then there is no issue at all basing whichever armies you are building to complement those.