Nice post Noises!
3 more roles for people to consider;
Flag Clearer means clearing the enemy flag of mines, shields, and other nasty deployables that are hazardous to a capper's health. Can be done by any of the heavy classes, or some of the medium classes like the Soldier and Raider. Can be done in conjunction with Gener-hating duties.
Heavy on Flag (HoF) is a heavy class usually with the Superheavy perk that is standing directly on top of the flag. Cappers coming in fast suffer a rapid (and messy) de-acceleration in a collision with a HoF, making it a useful defence strategy.
Anti-sniper can be either an Infiltrator or a Sentinal. Their role is to continuously harass enemy snipers to allow cappers to escape with the flag. Currently 2 Sentinals (if they are good) can lock down a base to the point where capping is nearly impossible. In most cases, consider Noises comments above in regards to the Sentinal class, however if you see 2-3 Sentinals on the enemy team and your Cappers are having issues escaping with the flag because of their rifle range, consider swapping to an anti-sniper role. Note latest patch has nerfed BX1 rifle a tad, so it remains to be seen if multiple Sentinals continues to be present on the servers.