First, I acknowledge this may be a bad idea as almost all players have a vested interest in this. Nevertheless, foolishly I venture my 2 cents.
Leech is a little boring and arguably overpowered because:
1. It ignores defence.
2. It ignores crit and associated stats.
3. It ignores elemental resistance.
4. It does fixed pure damage of 100% attack + bonus.
5. It restores fixed 40% attack + bonus to the hero with the lowest HP.
I suggest that leech be modified to the following instead:
1. Do pure damage of 2 x (sara attack minus opponent hero defence) x total opponent hero elemental resistance from soulstones + bonus.
2. Restores 50% of however much final damage is inflicted to the hero with the lowest HP percentage.
This modification would:
1. Encourage greater use of the defence stat.
2. Reward use of elemental soulstones with resistance.
I also advocate the transposition of such skill damage calculations (factoring both enemy defence and elemental resistances) to other hero skills. It appears that other skills do indeed factor this in one way or another (perhaps even smite and holy scourge), so it makes it even more strange that leech is the exception to the rule.