Ryan is pretty flexible in terms of tactics, so even a swap to Tim or Cassie wouldn't leave him totally out of his element, even if it does leave him less powerful. Plus there'd be a ripple effect as several other swaps are changed, and I don't think some of those swaps are as strong. I'm not sure Cassie should go into anyone besides Ryan, since doesn't have many other strong relationships.
Really, the Ryan/Anika switch has already produced their religious argument, meeting the entities, another argument brief argument about how her powers work, and should have some other fun stuff in the future. Like how Anika feels about Ryan using her powers to basically assassinate Mistress with a lethal-type attack. Or just the way he's using invisibility in general. Even if the tactics between the two characters can be pretty similar, I don't think it'd be easy to beat the interactions their switch has produced. Moving Ryan into someone with weaker might be a bigger change of pace, but I like the way things have turned out.
Of course, throwing Kelly into the body exchanges would also have made things interesting. Probably a little too interesting, since it would likely damage those plotlines instead of just mixing things up. A public recanting would likely totally destroy Kelly's credibility, and then I can't see her easily forgiving that person later.
