Yeah, we run a yearly convention (this is the 42nd year) and just switched the rules over to 5e. We did a series off playtests with the variants we wanted to best keep the feel including spell points, but we eventually discarded them because people were using a lot more high level spells, which had a commensurate bigger impact for the action economy, plus it lead to more 5 minute adventuring days as the casters used their big guns a few times and then were out. But we were looking for least common denominator issues, doesn't mean that's how it goes down at your table.
So the idea of introducing casting interruption is to have the foes interrupt the PC casters more than vice-versa, or do you expect the PCs to use it more often?
The main idea of introducing casting interruption is to change the way magic works in a way that makes it more pleasingly aesthetic to my old-school sensibilities. It's not a balance change per se.
But I do also want to alter the incentives for players thinking up PC builds, yes. A PC Fighter 1/Wizard X tank fighting in melee with Greenflame Blade has more reason to care about Warcaster, because disrupting his concentration also disrupts his attacks, and more reason to consider maybe just being a pure Fighter X+1 instead. A Paladin has a little bit of extra incentive to consider Wrathful Smite over Paladin spells like Wrathful Smite which do as much and more, just because at least Divine Smite can't be interrupted.
And I do want the freedom as a DM to create scenarios with evil wizards wherein the PCs can gain an advantage by separating the evil wizard from his guards. If ambushing Cthuchik the Mighty Evoker alone in his sanctum just means that Cthuchik is going to Meteor Swarm the 8th level PCs into oblivion before they can finish depleting his 95 HP, then that scenario is no fun and I shouldn't create it as an 8th level adventure. But if it's
possible for the PCs to kidnap Cthuchik and hold him ransom so that his army will abandon its assault on Tolnedra, then I can set up that scenario after all. I think this scenario is possible to run under PHB rules just by expecting the PCs to gag Cthuchik and capture his hands, etc., but if that's the only way to do it I find it a bit boring, and as mentioned it also doesn't fit my headcanon aesthetic for how magic ought to work. I like it if "Cthuchik mutters words of power and an eeries light begins to glow in the heavens, even though you're indoors--the light grows stronger and stronger and you can now see that it's aimed directly at you! He's casting Meteor Swarm" can be answered by "I punch him in the throat" as well as "I stick a sock in his mouth." I dunno, D&D is a game about violence, y'know? and violently interrupting a spell with violence is a thing that used to be possible, and that it seems like should be possible again in my 5E game.
If you're asking for my
assessment of who is going to get interrupted more often, I'll say: (1) PCs are more likely to be spellcasters, but optimized PCs will have ways to mitigate their concentration vulnerabilities and so they won't actually get interrupted all that often; (2) I don't use all that many intelligent spellcasters in my game normally, because I prefer a more low-magic/low-powered aesthetic that relies on MM/FeF/ToB/Volo's monsters than on a gameworld full NPCs who are all more powerful than the PCs--I like having the PCs be remarkable and heroic, not just shadowrunners doing a job for Mr. Johnson. So I suspect that overall, PCs will be more affected by the rule than NPCs and monsters will, but nerfing the PCs per se is not the goal.
5e's attitude is one of being good with radical rule changes. Isn't there any sort of AoO or spellcastus interruptus module in the DMG? (I'm starting to wish I hadn't run the Next playtest so much, it's left me perpetually perplexed about exactly what rules and modules actually exist in which form in 5e.)
