some other names that kinda popped into my head after thinking about this..particularly due to the fact they are well-known and DON'T TOUR the States (yet)
Pure Reason Revolution they were to play at ProgPower Europe afterall, and only have played a couple of shows on the East Coast + Nearfest in the past few years in the States.
Oceansize - hey..if you get Riverside to come, these guys have just as big a following if not bigger..they just did a special 3-nights/1-full album per night showcase in Manchester, UK ..But haven't toured the States since (and the only time) 2004 when they opened for some defunct punk band. There's a lot of fans in North America who are fans who've never seen them live. Reasons I imagine are finances mostly.