That, though, would make them too weak.(And they don't need another build time increase, they're already 40 seconds!)
If you're wanting them to be a army unit(to go with MM, which would be cool) they need to not be a harrass unit. Which would kill off what reapers are =s.
More health, less speed, more damage vs Non-light, cost more. Slow enough Zealots can catch them eventually. It would work, yes, but it would be a complete redesign(Which, honestly, they might need)