Honestly, adding one or two new enemy types isn't going to fix the problem. Although I would love to see the Berserkers from Scourge of the past become an enemy type in the fallen. The main thing that need to be updated is the current enemies within the game. Examples for each race would be Servitors in the fallen get an extended range in their immunity field to make them feel like the priority and push them all mostly into yellow bars, making them a more dangerous defence tool for the fallen. As for the Vex, make standard infantry work similar to taken thrall where if you aim down sights at them they teleport to evade you, they are supposed to be this highly adaptive machine race after all. As for the Cabal, maybe make the phalanxes more aggressive allowing them to push towards you with shields raised and having access to melee variants of them. Lastly hive thrall, change up their animation so they stumble and crawl, maybe even dodge out of the way of your aim.