This little gem of a houserule started out from a misunderstanding of the old 3.0 rules. I have since thought long and hard about it from many angles, and it seems to be a viable one to me. And in the groups I play with, at least, I've found AoOs to be a fairly rare occurance, so giving them a little more of the spotlight couldn't hurt too much (I hope!)
Basically, attacks of opportunity are spiced up by adding that moving INTO a threatened square can also provoke. Now, there needs to be an easy way around this, besides the 5' step or the withdraw action, something that adds a little more tactical use offensively. So, in comes charge. As is, charge is another rarely used tactic in my games, but if you say that charging only provokes for threatened squares you leave, suddenly everyone is trying to charge into combat!
All feats I could find with regards to either AoOs or charges, as well as some other things such as class or race abilities and spells, still work without any fixing, so from what I've been able to see, this houserule SHOULD be easily incorporated into any campaign.
If anyone can see something that is suddenly gamebreaking, or even seriously unbalancing, with this, please post!
