30-40-50% ms would change a lot in the healing department, and would warrant a different kind of peeling. And having a stacked ms with caster+melee could stop cleaves being the most powerful, which (imo) would make things more fun for sure.
This also wouldn't affect pve at all and would be a lot easier to work with for the devs in adjusting %.
don't think the current meta is right for a 50% healing debuff
15k flash heal casts for 8 seconds? u are forgetting heals without NS in wotlk were actually higher than they are now. a penance would tick 50% of someones hp with no MS effect and probably only does 20-30% maximum now. there was also no dampening effect when 50% ms was around