One of the big advantages I've seen that NPC casters have is that most of the time they have all their spell slots available at the start of a fight and no need to worry about saving them. So most of the time I open with their highest level concentration spell. If it requires a save and the target makes it, I can just cast it again next round.
For PCs it's not so easy. Our party just reached 7th level and the wizard took banishment. Great spell for taking out a foe quickly, possibly for the rest of the battle. The first time he got a chance to use it the target saved. It just seemed so...crappy.
So here is what I am thinking:
If a spell has duration and requires concentration and allows an all or nothing save, the caster can maintain concentration and on his next turn us an action to attempt the spell again on the unaffected targets as long as they are still in the AoE, within range, etc. The caster can continue to do this as long as he maintains concentration and as long as the duration has not expired.
Let's say a caster casts Hold Person with a 4th level slot (3 targets). Two of the targets make their save, one doesn't. Next round the caster can spend his action to reattempt the spell on the two that made the save (not on the one that he already affected...even if it broke out of the spell...
It is still going to cost an action. Just no slot.
