Re: Wild West
WGF have a sensible history of bringing out products in line with existing popular rules sets. WW2 US and Germans for Bolt Action, WSS and Zulus for Black Powder, etc. There are a number of Zombie rules floating about, though the Apoc Survivors work just as well as armed (or unarmed with minor work) civilians for any modern setting.
28mm wild west skirmish is pretty popular at the moment (Blackwater Gulch, Dead Man's Hand etc), and there are no other companies making plastics for it that I know of. The range would also be handy for VSF, Colonial and even ACW - typically wild west is 1870's.
Jim said a *new* historical setting, so I think we can rule out expansions to Colonial, WW2 or Ancients at this point. ACW is a definite possibility though... except you'd be going head-to-head with the Perrys. Likewise for Napoleonics... both Perrys and Victory have 28mm plastic pretty much sewn up. But both settings are massively popular and the market may well bear more players.
Otherwise.. 1635 is a good year. Musketeers, 30 Years' War and Piracy. And while Warlord have 28mm plastics for that era, there are gaps in their line to exploit (no plastic artillery for a start).