I think one of Tremulous's biggest missed opportunities was how thoroughly the helmet destroys the applicability of stealth for aliens. It's especially unfortunate because the helmet would be highly valuable for it's defensive buff alone, so players miss out on what would be an interesting decision between surviving more hits and seeing enemies before they strike. So something obvious that Unvanquished could do is split the helmet into two items: one that provides crucial protection to the head (or just a big defense boost if locational damage were to change), and one that gives a radar but no extra defense.
I recommend that, but I think you could also take it a step further by turning the radar into an opportunity for promoting teamwork. Now, in a match with perfect teamwork (i.e. everyone on the human knows each other, works well together, and has voice chat) the above would be enough: humans would only ever need one radar-bearing player per squad, and that person would just shout out when he sees an alien hiding. My opinion is that the shouting part of that is not what's fun or interesting, it's that the humans grouped together and contributed in different ways (radar guy by spotting aliens, the others by protecting his squishy head). Therefore, in order to extend that teamwork fun into matches with less than perfect coordination, the game could just automate the shouting: give a radar to every human that's near a teammate with the radar item equipped. I like this sort of natural teamwork encouragement where just doing the right thing (in this case, buddying-up with a teammate) has an immediate and obvious benefit.