I've done about the same thing a couple of months ago. The thread must be buried somewhere around here.
First, I made a feat called simply Power Swap, which has the MC initation feat as prerequisite. With this feat, you can trade powers up to half of each category (at-will attack, encounter attack, daily attack and utility).
Then, recognizing the potential problems in so much freedom, I placed a simple restriction: if you are multiclassing out of your primary role, then your highest-level power in each of the 4 categories must be from your primary class. For example, if you are a 3rd level fighter/rogue, you may have a rogue level 1 encounter attack but not a level 3 one; on the other hand, if you are a 3rd level ranger/rogue, you may have a rogue encounter attack of level 1 or 3 (although not both). After all, powers between the same role overlap reasonably, while those between different roles represent a much greater versatility.
The power swapping is valid throughout all 30 levels, but must always observe the restriction of not having more powers from the secondary class than from the primary one, in each category.
If you choose the Paragon Multiclassing option, your paragon powers must be from your secondary class as normal, and these also count against your limit of powers from the secondary class (see above).
In Paragon MC, starting at 11th level, when you use an action point, you may use an encounter power from your secondary class without expending its use, and may use it as if recharged if you have already used it.
Also, at 16th level, you gain a new class feature from your secondary class (I made a list of these).
Another thing that bothered me were the Pact Initiate and Arcane Initiate feats. Unlike other MC feats, they do not grant class features, but actual class powers. Also, I feel every wizard should begin his training with cantrips. So, instead of gaining a 1st level at-will power, he gains all cantrips (note he's just 1 feat away of actually trading one of his own at-wills for a wizard one). And the Pact Initate grants the warlock's curse once per encounter, along with the pact boon.
I realize the Arcane Initiate benefit is quite dull, but its a simple lack of class features. It would make little sense to give the Spellbook feature with the first feat, since at that point you can't learn wizard powers and should not be forced to get another feat (Ritual Caster) to enjoy its benefit. So there are the cantrips and implement mastery. As I said before, at 16th a new class feature is gained, and its usually a very nice one, so full implement mastery would be gained then. Thus cantrips with the first feat.
I'll post more about this later.