A single power swap feat
with 1/tier of swapping of enc/utility or daily, with a +2 to a single skill of the target class. Can be taken multiple times. (to retrain one out at level 11, and another at 21 if you only need three powers swapped). That way, you still need to "pay something" for that extra power, but the skill boost softens the blow and you can get access early if you are willing to (temporarily) pay that tax.
One thing I hate is the paragon multiclassing. With this variant MC swap feat, you could say you need one of those plus the base MC feat, to PMC, then you get the extra at-will as a bonus, not a swap (thanks for that idea! I will allow it in my Dark Sun campaign). Sometimes you just need to keep that at-will.
Then again, creating custom themes that are on the whole match your idea better would seem ideal. There are uber powerful class + power combos, even within the existing rules with those feat taxes, so it's better to at least get a way to allow certain combinations so long as they make sense together.