Could you let people protect allies using their Shield (Ground Assault and Slugga Boy) get a bit of XP when they block damage? This would encourage them to actually use their Shield to assist the team more often, which would be nice to see as it is a team based game after all.
Sadly, such an XP calculation is not likely to come in permanently, if at all it does. It‘s a lot of continuous proximity and line of sight checks.
As previously mentioned, we are looking at adding some new XP events along with personal goals. Specifically shielding allies as an event might be a heavy calculation on the server, however, so we wouldn‘t advise our players to expect that.