I particularly like @dvd4vr 's solution, though I would likely combine it with @jgsugden 's rule that one of the spells must target "you and only you", and probably a combined spell cap too. Fully writing this version out now so I can gauge and simplify.
I did, however, think there might be a simpler way of applying the main points of OP's Focus by just adding the following clause to the spells you think should be Focus instead of Concentration:
"Casting this spell does not break concentration on other spells. While concentrating on this spell, you may concentrate on one more spell than you would otherwise be able to. You may only apply this effect from one spell at any time."
And add "When you reach 5th level..." to the start to limit it a bit if you like.
If you want the level/proficiency bonus scaling from OP, (and I'm not sure I like the balance) then I'd do the following general rule rather than a whole slots system;
"Casters of 9th level or higher may cast any concentration spell without breaking concentration, and may maintain concentration on a number of spells equal to half their proficiency bonus, rounded down. If concentration is broken, it is broken for all spells that the caster is concentrating on."
Let that stack with the Focus spell clause, and you have a fairly close system to OP's, with the major differences being
- simplicity
- quicker access to 2*concentration + 1*focus
- a small nerf at high levels (you can only stack 4 lots of Shield of Faith at 20th instead of 6)
- immediate access to 1*focus + 1*concentration at levels 1-4, unless you add the min L5 caveat, in which case you can't stack anything below 5.
Slightly different outcome but I think maybe a little neater?
I did, however, think there might be a simpler way of applying the main points of OP's Focus by just adding the following clause to the spells you think should be Focus instead of Concentration:
"Casting this spell does not break concentration on other spells. While concentrating on this spell, you may concentrate on one more spell than you would otherwise be able to. You may only apply this effect from one spell at any time."
And add "When you reach 5th level..." to the start to limit it a bit if you like.
If you want the level/proficiency bonus scaling from OP, (and I'm not sure I like the balance) then I'd do the following general rule rather than a whole slots system;
"Casters of 9th level or higher may cast any concentration spell without breaking concentration, and may maintain concentration on a number of spells equal to half their proficiency bonus, rounded down. If concentration is broken, it is broken for all spells that the caster is concentrating on."
Let that stack with the Focus spell clause, and you have a fairly close system to OP's, with the major differences being
- simplicity
- quicker access to 2*concentration + 1*focus
- a small nerf at high levels (you can only stack 4 lots of Shield of Faith at 20th instead of 6)
- immediate access to 1*focus + 1*concentration at levels 1-4, unless you add the min L5 caveat, in which case you can't stack anything below 5.
Slightly different outcome but I think maybe a little neater?
