As long as both PCs and casters are limited to not tons of buffs, how would this be NPCs working differently? Concentration is actually a really good thing for limiting one of the biggest problems with NPC casters in 3rd edition.
Namely that they were _far_ more effective than non-casters. Not only because they had spells at all, which 3e overfavors a bit, but also because they could blow their entire pyramid on one fight.
The problem is not that they do not currently play by the same rules, the problem is that one of the reason I want to change the concentration rules is because of NPC casters that can't defend themselves adequately with spells. They need minions or helpers or what not. I want them to be capable of relying more on themselves. I like the idea of solos, but solo casters in 5E are probably just toast due to action economy.
So, 5e casters will have their whole pyramid available to them which is still a serious balance issue - having just done an entire mod of fights each with several scary guys and one caster where every fight was "who cares if it has a deadly poison super smash trample flying breath whatever. kill the caster first cause he can fireball _every round_".
It's already that way. If a PC (or NPC) pulls out Fireball, everyone is going to gang up on him today.
Which I guess suggests another thing you could do, which is have Concentration spells limit the casting of other Concentration spells to some extent. Perhaps you can cast a total spell level equal to your highest spell level. So you can cast 5th level spells? Great, one 5th, or a 3rd and a 2nd, or a 4th and a 1st, or 5 1st. Of course, a certain amount of madness lies that way too, but it's worth investigating.
That's one possibility. Thanks.
You could also charge extra spell slots for Concentration spells while you're concentrating already.
I suspect that this would be more of a detriment to PCs who tend to have multiple encounters per day than NPCs who tend to have one encounter, the one with the PCs.