well for the heal thing , i was thinking of healing wards, where people get healed only when standing on a relatively smaller ward. they would have to sacrifice mobility for healing , which could be a crucial decision when low on hp but in dire need to kill someone or do an objective asap. (this brings me back to my Terminator class that i wrote a year ago... im personally thinking that the Terminator race has a better overrall healing mode ...but still).
i like the "shooting teammates to heal them" idea. im sure there's smthn in the mod that can read "teammate shooting" event and allow healing effect instead of damage. This would actually suggest good team work, imagine that you have a tank player rushing and a support healing him with only a deagle to protect himself. And you get exp for every 100 hp you heal (equivalent to an average 100 dmg you might need to kill someone) ..This also might push players to not rely on speed races like spiderman, vagabond etc, since teammates wont be able to target them constantly. i like it.
another healing effect would be smthn like the molecule shield: but instead of invincibility, it creates a bubble that heals you and teamates near you but you cannot move until its over.

as for the engineer, i was thinking of shit like the half life laser trip mine that can be planted around corners to surprise players walking by, or like trigger grenades. : you get an HE grenade every 10 seconds that sticks to where you throw it but doesnt explode (max 5 at one time), and when you activate +ability it detonates them? making this race more of a tactical race that relies on knowing the best spots in the map to plant some pre-emptive traps and waits for its target to pass by. its strength would be in its SHITLOAD of hp (more than 1 nebula can proc) but with HUGE knockback if it gets hit to avoid using the race to plant and defuse bombs at ease