Meh, even per wizard it'd be awkward the moment you have more than 1 available cast. I like how they do it with Kroak's celestial deliverance getting progressivly harder to cast. A mechanic like that would have my preference as long as they can manage to find some balance in between "too easy so certain models with excessive acces to modifiers succeed anyways" and "too difficult, so without modifiers getting more than 2 casts off is basicly impossible". The way the new arcane bolt works, being more effective with a better casting roll, also helps curb abuse simply cuz if it succeeds multiple times, a bunch of those are probably not going to be too impressive.
Meh, as long as Nagash retains his ridiculous volume of spellcasts and his modifiers he'l Always break any halfway interesting magic rules. A model that has 9 spellcasts when the average wizard has 1 and a +3 bonus to all of those spellcasts is just ridiculous… The first thing they should do if they want to make magic interesting is nerf him into the ground (or at least, his casting abilities). I'm not surprised he's one of the prime reasons for the rule of one.
... sadly there are even more lovely models i am not able to buy 
