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eamon : On your AC points, keep in mind that this particular campaign uses inherent bonuses and it sounds like the DM is controlling the loot, so PC won't be able to boost their armor class by picking "the right gear" as you would see on the CharOp boards.
This is only true to a degree however. First of all, I'm kind of assuming the DM isn't out to get the characters, so that to some extent they will get what they want. Secondly, you may be thinking "cheese" when you hear charop, but the reason I refer to it is simply because it's a large forum with lots of ideas. Even without
any say in magic items I'll bet you'll find decent ideas over there. E.g. the parrying dagger I mentioned earlier: that's just a superior weapon; the Mage's enchantment merely means you don't need the proficiency feat for it. And there's probably
lots of other stuff around there; it probably won't add up to much (4e is fairly balanced), but you'll easily surpass 22AC anyhow. So the point then is: are you specifically out to limit such combos?
In general, this really depends on the campaign. If you intend to strictly limit all AC-boosting enchantments; that's fine. And as with all such campaign-specific rules: just make sure the player's know
before they build their characters.
That's why I'm so leery of where this thread is headed. I see at least one PC
specifically built to have a high AC and be very hard to hit with OA's. That's his thing. And he's doing it without being game breaking or even unbalancing: as discussed before, the hit rate on non-OA's for level-appropriate enemies is well above 30% and will rise further as he levels; this is OK. And there's another PC who is playing a by-the-book defender. You're suggesting to alter the game (and obviously
after the campaign has started), to nerf these characters
for no good reason! Now, I'm not in your campaign; I don't know it nor your players, and it's very possible none of this is relevant. But there's certainly no motivation for the change in this thread, so this looks like a case of arbitrary-DM-nerf.
Don't pull the rug out from under the PCs. Not unless you have to. Basic attacks tend to be fairly weak, do only damage, and be versus AC. Shaking up the game can be fun. But your changes look like they're for all the wrong reasons. You look to be punishing a player choice that's non-problematic, and without warning beforehand.
You're asking "How can I mitigate the high AC of these PCs?" - when you should be asking "Should I nerf these PC builds?" - and you should not; they aren't problematic, and you're violating the cardinal rule that player choices should matter.