I doubt this will be popular. I'd rather see WF build on what they already have made, rather than a complete new range. Hence, heads, weapons and maybe some body types so as to make other african tribes including late Zulu warriors. Some shirt and pants types, spears, bows, shotguns, shields, different tribal heads. This would expand the range greatly with only one sprue.
Once that is done. Then continue with the theme with Natal, and Boer types, including mtd. troops. In other words finish a complete range of African Colonials. Then go from there. Should be able to handle it with a maximum of 3 new sprues.
For a complete new range. The rebellions of India would be my vote. Hasn't been done overly much and is quite colorful. Plus, who doesn't like Elephants?