What kinds of hands limp? And, how many of those will call a decent raise? I'm not so much advocating a PF raise as I am interested in investigating it. I'm pretty curious about limpers and limping in general and think there may be something to exploit here.

A lot of the discussion around limping indicates that low and middle pocket pairs are common limping hands and that those who limp with them will often fold to a raise (even though they may be ahead of something like ATs). And, lots (not all) of hands stronger than ATs will already have raised. So, ATs maybe be ahead, and want to thin the field.