They've unlocked things with reckless abandon before (scrolls, ba hide, ba disengage) so I'm expecting some terrible unbalanced mess with caster multiclassing after their statement.
The worst would be allowing max spell level for all caster classes be determined by character level or highest caster level. But those would also be the simplest and I can't see them making the rules more complicated than 5e. Probably character level so players can't "ruin" their characters by going Cleric 6 / Wizard 6.
We will have Cleric 11 / Wizard 1 casting Disintegrate or something asinine like that. Single class casters rendered obsolete.
And why exactly does the game need this rule change when you can already respec at will?
This is what I'm worried about the most. We have respec so they don't need to be ruining the balance between single/multi class. Also, I'm assuming the level up screen for multiclassing will work similarly to choosing your subclass. In that it will show you what you would get as you choose a different option. If I stay wizard then I'll get fireball, if I go something else then I lose don't get that spell this level.
The pros and cons are what makes building your character fun. I don't want multiclassing to have all the benefits with none of the drawbacks.
