I really like the Concentration mechanic as-is, but I could see an argument made that a few spells that are Concentration should not be. For those, I'd be tempted to instead add "if you cast this spell again while the first spell is still active, the first effect ends immediately". Maybe keyword that to "Unique"?
I think the '
Unique' is a good start, but that could also be paired with an intermediate version of Concentration for spells that don't require you to actively control them. "
Maintained" could be a good word.
"You can maintain any number of spells, as long as their total spell slot level does not exceed your Highest spell slot level + your casting modifier. These spells immediately end if you are incapacitated, or if you spend more than one minute on another plane of existence from them." You could even have Concentration spell levels count against your Maintained limit.
This gives you
Maintained for spells like E
lemental Weapon, the S
mites,
Hunter's Mark,
Hex, and most
Conjure <insert monster> spells; things that you're powering, but not actively controlling. That leaves
Concentration for things like
Flaming Sphere or
Major Image., where you're actively directing them.
For the problem of stacking smites, just have those spells explicitly state that no creature can be affected by more than one smite spell at a time. If another smite spell is cast on it while one is still going, the smite spell that was cast with the highest level spell slot wins.
Alternate-alternate option:
Willingly dropping concentration doesn't cancel a spell, the spell just becomes 'inert' for lack of a better term. You can resume your concentration on it later, so long as it still has time left. For example, a Paladin can be concentrating on his elemental weapon spell, drop concentration on it to cast a Smite spell. After he's done concentrating on the Smite spell, he can resume concentrating on the Elemental Weapon spell again for whatever remains of its hour, without having to re-cast it.
Losing concentration on an active spell due to damage would cancel it totally, as per current RaW. Being incapacitated would cause you to loose all 'unpowered' concentration spells as well.