[Switchblade]

I'd bring over Blade. It's a show with a large, but not overly stuffed, roster of Riders that Bandai of America could make crappy action figures out of and the weapons and different belts would make for good roleplay toys. There's maybe even some form of CCG in the card gimmick if someone wanted to exploit that.

As far as the story goes, I would largely keep the same plot and characters from the Japanese version, but probably quicken the pace of the first block of episodes (I'm assuming this would be a ~30 episode series and not 48 anyway). The only major thin I would change is Chalice's gender - I'd make him female in this version so that we could play up the Blade/Chalice relationship as more of a romance, which I think would make the ending work more powerfully for an American audience.