If you are prepared to look beyond North America there's a wealth of possibilities for ECW type figures, especially if you aren't overly hung up about figure accuracy.
The 1655 Cromwellian invasion of Jamaica is of course the perfect setting for ECW troops in the Americas. Heading further south you have the Luso-Dutch struggle for North Eastern Brazil which apart from offering a range of skirmish actions in an exotic setting with just as exotic allies, also saw some decent sized battles, like Guararapes which remain within gameable scale as well as some decent sieges, like Salvador (twice) and Olinda. ECW troops would make suitable proxies for both sides.
Of course the Brazilian campaigns are just a theatre in a much wider global conflict, probably the best claimant to being the real WW1. You could fight in Goa, Angola, Sri Lanka and across the islands of South East Asia.
Moving to a somewhat earlier period, there were the Portuguese campaigns to remove vestigial pockets of French influence in Brazil at the start of the 17th century. With ECW troops you could expel the French from São Luís.
Anyway just some more suggestions.
If you fancy doing King Phillips War be aware that there is a nice little range from Brigade games you could use to supplement your ECW troops with. The war falls a bit outside the usual scope of ECW uniforms but you can always claim the damn colonials were slow to adopt current fashions.