I think the Etherform (those color-balls you turn into when dead) is a really unique and innovative way to deal with respawn delays, however I think they could be still improved a bit further to make them even more integral to gameplay.
One recent idea I had is to make them transparent/invisible based on player distance, i.e. when a Etherform moves close to a CAT it first becomes more transparent and then is completely invisible. This way they can't be used to distract players when spectating close. But it could be also an interesting defense strategy as respawning players could "hide" near their CAT form team-mates and spawn for a surprise attack on enemy players in their territory.
The other idea I had (and this is based on rush strategies from Savage2) is to have a sort of deployable structure that allows players to spawn (with some penalties?) in enemy territory if placed there by another player of one's team. Obviously these structures need some health so that they are an priority target for destruction by the enemies. Visualization could be as a small energy bubble of owned zone within the enemy zone
Some way to effect the energy recharge in the zones might also be interesting. For example the further you are back in your territory the faster it recharges, or certain structures in the game allow quicker recharge if the zone is owned and you hover nearby or something like that.
Any other ideas?