So according to LoL Wiki health value is around 2.67 gold per point and mana value is 1.4 gold per point.
Therefore I propose something like this:
1st or 3rd row of Resolve tree:
Mind Over Matter
50% of your Bonus Mana is converted into Bonus Health (1 point of mana = 0.5 point of health).
100% for manaless champions.
ex.
250 mana = 125 mana + 62.5 health or 125 health for manaless.
I would not include mana regen here thou as it could result in abuse from
or
passives and the only items that have any "standard" mana regen are already support items anyway.
*Edit.
Lets check out items and see if thats not too impactful:
: +75 hp // +150 hp for manaless,
(no stacks) : + 162.5 hp // 325 hp for manaless but no mana to ap passive,
: +350hp // cant be stacked by manaless (no mana expenditure),
: +75 hp // +150 hp for manaless,
: +100 hp // +200 hp for manaless,
: +62.5 hp // +125 hp for manaless,
: +150 hp // +300 hp for manaless,
: +125 hp // +250 hp for manaless,
: +62.5 hp // +125 hp for manaless,
: +150 hp // +300 hp for manaless,
(no stacks) : +62.5 hp // +125 hp for manaless,
: +250 hp // cant be stacked by manaless (no mana expenditure),
: +50 hp // +100 hp for manaless,
: +100 hp // +200 hp for manaless,
(no stacks): +75 hp // +150 hp for manaless,
(max stacks) : +100 hp // +200 hp for manaless,
: +62.5 hp // +125 hp for manaless,
: +62.5 hp // +125 hp for manaless,
(no stacks) : +62.5 hp // +125 hp for manaless,
(max stacks) : +250 hp // cant be stacked by manaless (no mana expenditure),
: +25 hp // +50 hp for manaless,
: +62.5 hp // +125 hp for manaless,
: +62.5 hp // +125 hp for manaless.
All in all this doesn't look very abusable. We need to remember that you would need to sacrifice a minor runeslot for this (in Resolve tree on top of that) so imo it's not a bad idea. Hell we could probably even bump it up to 75%/150% for manaless and it still would end up being just an ok rune with all the impact other runes have.