I'd prefer the syringe to be as good for everyone as it is for medics so that it's somewhat viable to run without medics in the distant future where 10 perks exist.
It would be nice to have some alternate options so you aren't required to run 1-2 medics on HOE / Suicidal. That is probably pretty hard to balance though.
Improving the healing syringe for non healing classes would be a start. Maybe a 20-25% boost to its effectiveness. Really a matter of keeping it from being OP.
It would be nice if one of the other perks was built to be a good dual threat healing/damage class as well. Would be cool to see a perk that drains health from the zed in some manor and transfers it to teammates in a radius. Like a DMG-> healing in a radius perk or where you can absorb energy from dmg and transfer it in some way to damage / healing.
Or possibly like how a buff banner works in TF2 where a perk builds up rage and once a bar fills up they can channel that energy into a one time use aoe heal / aoe dmg ability.