I think 5e will have to walk a very careful tightrope because you don't want taking a level in a new class to give too much or too little. Unfortunately, it appears that simplicity (or alternately, rules elegance) will be the casualty.
I've always been of the view that one of the key reasons why spellcaster multiclassing was problematic in 3e was because gaining one spellcaster level led to gains in three or four dimensions:
1. An increase in the number and power of spell slots available (this can be further decomposed into the total number of slots and the power of each slot);
2. Access to more powerful spells, about every other level or so; and
3. An increase in the power of individual spells.
What we're seeing at the moment is a healthy dose of both 1 and 2. Wizards are quadratic (I'll argue that before they were cubic!).
The problem is, gaining a level in a non-spellcasting class gives you perhaps a bit more to-hit, a bit more damage, a special ability. Gaining a spellcasting level gives you more spells and better spells. In the simplest case, a level of Rogue gives you another +1d6 on sneak attack, and you can do that as frequently as you previously could. A level in Fighter gives you slightly better, perhaps more, expertise dice, which you can use as frequently as you could before. A level in Wizard gives you more frequent use of your existing spells and you get new, more powerful spells.
To fix this requires a fundamental shift in how spellcasting progresses. It needn't be the one size fits all approach of 4E, but something has to change.
If we had to keep 9 spell levels (which is hard work), then I would attempt to detach the ability to cast them from Wizard level. Perhaps return to the requirement of 10+spell level Intelligence to cast a particular spell. Then I would have a spell-point-like preparation system, so that a Wizard gets an increasing number of spell slots available, but higher level spells take up more slots. A Wizard would have an entry in their class table for 'spell slots' and 'maximum number of spells', but spell level would be detached (hence this would be much easier if there were fewer levels and they could be curved less dramatically). Getting a single level of Wizard might let you cast a pretty good spell, but you'll just have one, or you can have a suite of lower level spells. The only problem is preventing super-spells being available immediately (and my suggestion here is returning to the days of no automatic spell learning - these things exist but must be earned!).