Healing returned by Lifsteal shpuld be capped by the target's remaining health.
It's really frustrating when an ADC crits a minion that has barely any health left and heals 1/4 of their health. If the target only has 50hp left, then lifesteal should only return a maximum of 50hp from hitting it.
They'd still be able to fully heal off of a fresh minion wave (and probably even half a minion wave). And their lifesteal vs champs/monsters would be pretty much unaffected. They just wouldn't be able to heal themselves more than they should be able to considering how much health their target actually has.