Do you have specific examples of spells from your spellbook you would never prepare at the same time because they are both concentration spells from the same level? Or examples of two concentration spells you would only ever select one of when leveling up?
Level 2 spells for sorcerer or wizard:
Alter Self
Detect Thoughts
Enhance Ability
Invisibility
Suggestion
Spider Climb
A large variety of infiltration/social interaction scenarios would want to be able to use more than one of those spells at the same time. Sometimes you might be able to get away with recasting the lost spell (and feel like it's a waste of a spell slot), but other times not (particularly when using Alter Self or Invisibility). Mostly, though, you ditch the extra spells. If you do prepare them (as a wizard), it's for a different intended uses. As a sorcerer, you only pick one or two of the most useful, because you'll never be able to combine them anyway.
I see five basic purposes for concentration:
1) The spell can be manipulated and changed over the course of its duration. Concentration represents the focus needed to be able to tweak it from time to time, rather than leave it in a 'fixed' state. An example of this would be Alter Self.
2) The spell is theoretically manifesting magical power that must be continually maintained. This would be most common with conjuration spells (why are most wall spells Evocation instead of Conjuration?). An example of this would be Fog Cloud, but True Strike is likely also in this set.
3) For game balance reasons, the devs don't want you to cast the same spell over and over again. This is likely the case for buffs like Haste.
4) For game balance reasons, the devs don't want you to cast this spell together with various other spells that, together, lead to an overpowered character or party. An example of this would be Fly + Greater Invisibility + Stoneskin + Whirlwind + etc.
5) (Edited addition) Make it possible for someone to interrupt your concentration to end the spell. This is almost exclusively within the realm of spells maintained during combat. It's the only way to end buffs, whereas debuffs can generally be ended if you make a successful save. Being able to do this to the enemy can be important.
So 1 and 2 are thematic reasons, while 3 and 4 are game balance reasons.
We also have one other mechanic that fulfills a similar purpose to #3, as seen in the Light spell: This spell ends if you cast it again.
So spells where you don't want multiple instances of it being cast, just add a termination clause. On superficial review, I'd apply this condition to all concentration cantrips — Create Bonfire, Dancing Lights, Friends, Guidance, Resistance, True Strike.
Of those, most seem to be type 3, so just add a termination clause to prevent spamming them. Only Dancing Lights fits type 1, and it's not much different than Control Flames (which can alter flames for a 1 hour duration, rather than 1 minute) or Mage Hand (which also has a 1 minute duration, but no concentration requirement).
Another potential mechanic that could be added is to change it so that, when you cast a new concentration spell, any previous concentration spells you were maintaining are just suppressed instead of terminated. That way you can have something like Hunter's Mark, which lasts an hour, and not lose it because you wanted to cast something like Ensnaring Strike or Zephyr Strike. You just don't get the benefits of Hunter's Mark while you're maintaining the other spell, but you don't waste the spell slot from the original cast, either.
That's more bookkeeping, though, so it may have been avoided to keep things simpler.
The number of concentration spells goes up dramatically once you get into leveled spells, so it's a lot harder to judge them. But I can throw in a few things that seem likely.
I would probably just add a termination clause to Haste, since it seems to be a type 3.
Bane and Bless seem to be concentration to explicitly counter each other. Bane would also be a nice combo with any spells that require saving throws, but is it more overpowered to combine it with Slow than with Fireball? I would probably combine Bane and Bless into a single spell, and you have to choose the effect when you cast it. Then just add a termination clause. There's still a slight justification for concentration, but it would take more time to figure out if it's truly needed.
Pretty much any damaging spell that uses concentration should keep it. Allowing multiple maintained damage regions quickly scales the mage's damage potential way too high.
You shouldn't be able to stack defensive buffs (Fly, Invisibility, Stoneskin, etc), but at the same time, concentration feels like overkill. Why does Warding Wind lock out the ability to use Slow or Sleet Storm or Fog Cloud or whatever? Unfortunately, there's no simple way to say, "You can't have more than one of [these] spells active at a time" separate from the concentration mechanic.
Maybe, "You can't cast more than one buff on any one person."? But then you can have one mage with Warding Wind on himself, Haste on the barbarian, Bless on the thief, Fly on the Cleric, Invisibility on the bard, and still able to cast Slow on the enemy. (And does Mage Armor now count? Whoops!) Of course he's burning through spell slots, and it would take forever to get all that in place — which just leads to the prep-stage, before-battle casting from earlier editions that was such a headache and time-eater. Probably easier just to keep the concentration mechanic.
Anyway, that reduces the number of concentration spells to examine, but the ones that are left are still difficult to judge. Detect Evil and Good? Detect Magic? Detect Poison and Disease? Expeditious Retreat? Fog Cloud? Alter Self? Barkskin? Calm Emotions? Enhance Ability? Locate Object?
It's easy to see
why these were set as concentration spells, based on the above categories, but do they
need to be concentration? Must they fundamentally not be cast at the same time as each other, or with other spells? Would a termination clause be sufficient to limit excessive use?
I'm kinda thinking a termination clause would work for most of them, and other similar spells. But any mechanic outside of termination and concentration seems to quickly run into untenable problems, or become too complicated to mesh with the system design.
