I've been experimenting with altering the concentration requirements in my current campaign.
The reason is that the way it's currently setup means a lot of potentially interesting spells never get used, because you have save your concentration works for only the best of the best spells. (And my players don't even super-optimize!)
First I made sure I understood what concentration was doing. It covers 2 different things:
1) It prevents the hassle of pre-buffing the party with lots of long duration spells. If you've played any 3e, or most D&D video games of 2e-3e, you are very familiar with this issue.
2) It allows for spells to have end conditions, so debuffs are escapable, and buffs can be overcome.
These considerations--especially the second one--apply to both the PC and opponent side of the battle. So they aren't about party vs foes balance, but about spellcasting versus anything else. That being said, I have other ways of boosting non-caster PCs, and my veteran group has rarely (and by that I mean never that I recall) had a problem with feeling that non-casters are overshadowed. I know that will sound mind-boggling to some of you, but it's not unusual amongst groups that accept that the world-changing non-combat stuff in D&D is exclusively for caster classes, and don't involve dedicated optimizers treating character building as a mini-game to be won. In that context, I'm not be afraid of experimenting with things that might boost spells a bit, and also quite interested in threads like this to see potential issues I haven't seen.
On to my experimental rules.
A) In the case of spells with a 1 minute duration, issue #1 is completely irrelevant. Outside of an occasional ambush situation, pre-buffing is not a thing, so you will typically only have the spells up you cast during combat itself. Even in ambush situations, you are limited by the ticking clock duration (and your number of spell slots in adventuring days where you expect more than one battle).
Therefore, experimental rule A is (brace yourself): You can concentrate on any number of spells with a duration of 1 minute or less.
Part of the logic here is also that the opportunity cost of casting concentration spells in combat is competing with other spells. Sure, you can spend 2 rounds of the average 3 round combat setting up buffs and debuffs, but those are rounds you aren't casting fireball, and for each round after the first a spell with a duration becomes less powerful, since its potential duration is less. I'm very interested in seeing how this works in practice.
B) When you make a concentration save, it applies first to the spell with the shortest remaining duration. If you succeed, stop rolling regardless of how many other spells you are concentrating on. If you fail, lose concentration on that spell and roll for the next. Repeat until you succeed or are no longer concentrating on any spells.
This one took quite a bit of thought. I didn't want a single failed save to eliminate all spells (although I think that method would work fine with my other rules, and is a potential change I could make if they turn out to be overly strong). I also didn't want a save to be made for every spell on every instance of damage, because that would be a hassle. I set this as the spell with the shortest remaining duration, because it prevents you from being able to set up a "buffer spell" that you don't care losing concentration on. You just can't reasonably plan that with a longer duration spell, which means that it will almost always be the first concentration spell you cast in a combat (ie, the most impactful one) that is subject to the first save.
C) When it comes to how many longer than 1 minute duration spells you can concentrate on, I initially tried only 1. However, that proved unsatisfying in play, as there were more bread and butter spells with longer durations than I was expecting. I spent a good deal of thought coming up with alternate rules, some of them I really liked, bt added complexity. The one I'm currently experimenting with is: You can concentrate on no more than 2 spells with a duration of longer than 1 minute.
These spells are subject to rule B just like any other.
It's simple, but I think, given the other considerations of play (limited spell slots in 5e, concentration saves), it should handle consideration #1.
D) Finally (although I actually did this part first), I did a comprehensive analysis of which spells did and did not require concentration. After I figured out what basic rules they seemed to be following, I decided to make a few changes, which meant some (not actually a huge number) of spells lost their concentration requirement. This included some debuff spells that both had concentration and allowed a free save to end at the end of each of your turns. I generally removed the requirement on spells with an effect up to and including incapacitation. Spells with a greater effect (such as ones that turned the target against their allies, made them act randomly, etc) retained concentration. I also removed concentration on a select few buff spells that were so weak they were never taken, and seemed in line with similar 2024 removals. I removed concentration on some area effect spells, but never any that caused ongoing unavoidable damage.
These changes targeted spells that were still just not something my players ever wanted to use, because they felt they weren't worth it, and my other rules didn't really fix it.
So, assuming things work out as I hope (we'll see!) the outcome should be that just about every spell in the game that isn't just straight up too weak (and when I see those I try to fix them individually) should be a good option to take. Payers new and old should be able to just pick a spell that seems cool based on the non-mechanical description and not end up taking a trap options that will weaken their character. Some spells will still remain a bit more powerful (perfectly balancing every spell is impossible) and/or generally useful, but the fact that you aren't limited to picking just one to concentrate on, and that the balance between them has been improved (I run 2014, but adopted a lot of the 2024 spell rebalances) should make almost all spells good choices that can actually get used by players with experience.
