Problem is we also don't really want to bloat the rare pool even further by creating more chips to help cater for all the other classes and setups out there. That's kind of why the +5 INT on the old SF worked, because while it wasn't hugely useful to a spy it also wasn't a hindrance to them either.
It was however beneficial to PE's getting them within reach of medium belts. I would imagine that it possibly also had some benefit to pistol/rifle using tanks as well. (I also quite like the idea of something like +3 DEX on the Marine but that chip really needs it's STR stats fixed first).
However, if you want to have more 'specialized chips' out there (which actually I think I'd like to see, but see my point below) which require/give people more implant choices then I really think they should do it through the Faction Databases.
I think they should make all level 5 implants available through Faction Databases again, but make them reasonably difficult/tricky to get.
I think would balance things, as it gives everyone plenty of options regarding implant choices and will also benefit us by leaning/taking some stuff out of the rare part pool in the process.
Oh, and I think the malus on implants probably needs looking at as well.