Hey Miim, long time no see
Interesting thoughts - I guess the problem with making Drain Life a viable source of DPS is thats its healing component would need to stay as is, or risk it becoming overpowered (I can hear the PvP QQ's already) but a new, seperate spell (like mind flay without the slow) could be implemented.
Allowing a DoT tick to crit is potentially horrific in my opinion - they allowed it for Death Knights who's spells recieve a far lower coefficient but for a typical affliction Warlock sitting on (at a guess) 1500 damage at level 80, with the huge array of available DoTs then it could easily overtake ruin. It itself thats no bad thing if affliction Warlocks are really disgruntled with having to use a Destro Filler but I get the impression that as the community has been clamouring for that particular change only to see it finally go to a seperate class that its not going to be implemented anytime soon - if at all.
Perhaps a Deep Affliction talent in the same way that the DK talent is deep unholy would be workable.