That's pretty much my thought - but I think he'd still need to pay something extra in eschange for all that versatility. Giving the current (balanced?) Wizard access to a bunch of other classes' powers empowers him, so he's gotta lose something somewhere.
I did consider writing a supplement called "I want my Vancian magic back!" which was pretty much a translation of the 3.5 wizard to 4E, complete with the whole memorization subsystem, and every 3.5 spell translated in 4E terms, but it seemed too complicated to do in terms of balance.
Warlock and Wizard use different statistics for their powers. Maybe that's a way to "balance" them? Of course, Artificers and Swordmage seem to work differently already, so it's just for this specific combination it can work.
An other alternative might be:
The Wizard can use other classes encounter powers as daily powers only?
1.)
Remove the Arcane Implement abilities.
2.)
The Wizard gets one extra Encounter and one Extra Daily power each day.
He can use the encounter slots to fill them with any at-will attack OR utility power from the arcane power source.
He can use the daily slot to fill them with any encounter attack or utility power from the arcane power source.
He can spend both slots to prepare one daily arcane utility or attack power with the arcane power source.
3.)
A Wizard doesn't gain any non-Wizard spells by default. He has to learn them and write into his book. This might require buying a "scroll" of the power. (I suggest prices like equivalent level scrolls or potions?)
4.)
A feat that might replace Expanded Spellbook allows him to prepare at-will powers from other arcane classes as Encounter powers, and encounter powers from other arcane classes as daily powers, in both cases using his "regular" slots for them.
This is a benefit most wizards might not be all that interested in, since it probably weakens his abilities.
5.)
If the Wizard multiclass in any class with the arcane power source, he can prepare encounters as encounter powers and daily powers as daily powers with his extra encounter/daily slot. If he uses Paragon Multiclassing, he can treat all powers of the arcane class as wizard powers. (So at each power level, he can select any powers up to that level he knows from his wizard list and his multiclass list of powers. This benefit probably shouldn't apply to the power slots gained by Paragon Pathes/Multiclassing)
And a Paragon Path might instead expand his abilities to treat powers from a different power source similar to arcane powers. (Especially Divine, Primal or Shadow might make sense.)
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Personally, I am not a big fan of Expanded Spellbook and other feats that give you more powers. It creates a big laundry list and you still use only a very select few. It bloats the character sheet, too.

But I like coming up with rules
