Have healing skill grant a Regen effect to all cures (whether it's Cure, Cura, or Curaga), with duration and potency both based on healing skill. Allow this to stack with Regen spells.

As far as where tiers would land on this, I'd go with:

Cure - 1%/3 skill, 1 tick/4 skill, caps at 10%/10, starts at 1%/3
Cure II - 1%/4 skill over 20, 1 tick/5 skill over 30, caps at 10%/12, starts at 2%/6
Cure III - 1%/5 skill over 50, 1 tick/6 skill over 70, caps at 12%/15, starts at 3%/9
Cure IV - 1%/6 skill over 90, 1 tick/8 skill over 120, caps at 12%/20, starts at 4%/12
Cure V - 1%/8 skill over 160, 1 tick/10 skill over 200, caps at 15%/30, starts at 5%/18
Cure VI - 1%/10 skill over 240, 1 tick/12 skill over 300, caps at 15%/45, starts at 6%/24

Curagas cap at the same point as their equivalent Cure (Curaga = Cure II, etc.).

Cura - 1%/7 skill over 120, 1 tick/9 skill over 150, caps at 5%/30, starts at 1%/5
Cura II - 1%/12 skill over 270, 1 tick/5 skill over 330, caps at 5%/60, starts at 1%/25
Cura III* - 1%/18 skill over 350, 1 tick/3 skill over 400, caps at 5%/90, starts at 1%/45
(* This is obviously assuming that we get Cura III, which isn't a done deal by any means.)

The cap, including gear, at 99 is going to be somewhere just north of 500 skill, so the only spell that isn't capping on duration would be Cura III (based only on existing gear and assuming that 96-99 is 7 skill/level, which gives 511 skill fully geared and merited, it'd be 5% for 82 ticks).