I like this. I have no qualms about adding a new bonus type to keep this thing in check; if we only use it for this special case, what's the big deal, right?I know there's supposed to be a fixed list of bonus types (ok, I looked it up: armor, enhancement, feat, item, power, proficiency, racial, shield) but I don't see a problem with adding another.
Why don't we make two changes:
1. Weapon Expertise and Implement Expertise provide an expertise bonus instead of an untyped bonus.
2. All characters get a +1 expertise bonus to all attacks at Level 5 (+2 at level 15, +3 at level 25).
Then the Expertise feats automatically become useless at level 5 without any special rules, just because the bonuses don't stack. People don't have to retrain them, but they're probably going to.
I think I agree with the sentiment on this, but it is kind of weird that a feat suddenly changes purpose at level 5, isn't it? I think the 3/6/9 bonus sounds good, but I'm not sure Expertise should switch purposes like that ... thoughts, anyone?If we do this, I'd probably give Expertise feats some other minor bonus that kicks in at level 5, so that there's a point to keeping them. Maybe +2 to damage on critical hits with that weapon or implement at level 5, +4 at 15, +6 at 25. (Although that would encourage a weird retraining progression of its own: keep Expertise until level 11, then retrain it to Devastating Critical until level 15 or 25, then retrain back to Expertise to reduce the variance in your criticals. Or just take both.)
I picked critical hits because it's a bonus that won't come into play with every hit, so it doesn't completely change the feat, and +2/+4/+6 because it seemed like +1/+2/+3 damage wouldn't be worth it. Maybe +3/+6/+9 would be better? Comparing to Devastating Critical, which does avg. 5.5 damage, that means from level 5-10, this is infinitely better (can't take Dev. Crit till level 11), from 11 to 14 this is much worse (3 vs. avg 5.5), from 15 to 24 this is competitive (6 is almost equal to the avg damage, with no chance of low rolls but no chance of high), and from 25 to 30 this is much better.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.