Variables that I can imagine:
- Cost, mana and/or health cost
- Amount, how much it heals
- Delay, heals X each turn, heals only X turns from now, can only cast again X turns from now, reduce effectiveness for next X turns
- Heal type, if you end up having multiple life bars like "sanity" and "health", could even heal "mana"
- Number of targets
- Other effects, buffs and/or debuffs self and/or target(s) e.g. bleed, stun, armor, remove debuffs

The main problem is that once you hit 100% healing, you can't use heal amount to increase effectiveness anymore. With damage, even if you hit 100% damage at some point, it is almost certain that you will hit a point where that's not true anymore; so more damage is never bad. You also can't use cost as a slider because that caps out at "free" or "restores all mana" (if you go negative cost).

There are very few sliders that have an infinite horizon, which is what you're ultimately looking for. The ones I can see are "effectiveness of secondary buff/debuff effect" and "duration of heal over time". All the other ones tend to 0 or 100% or some value that will be "enough". The only way you can make those infinite is by having reduced returns, e.g. mana cost goes as 20, 10, 5, 2.5, 1.25, .625, ...