How about something covering native american indians.
For those that like the English civil war and upto Napoleonic period I can recomend CDV's Cossacks series, though the campaigns are not in the same style as RTW.

As for improving the RTW game, I would like to see sieges more realistic with maybe increased time before a blackade is fully in place, and maybe basic local troops being raised with zero recruitment time to bolster the defences but which returns to being civilian after the battle (if they survive)

First World War would be cool too