I'm not sure it this is really a direction that Trion is thinking about taking but I think it might be interesting to occasionally have multi-invasions in a zone, where 2 otherworldly forces attempt to take the same zone. For example, Air and Fire trying to both take Silverwood.

Dynamics ideas for this:

The factions in the zone could temporarily ally with the smaller of the two forces for the duration of the invasion. "Enemy of my enemy" kind of scenario. If the larger force is defeated, then a scenario would occur where you'd remove your former ally.

An opportunity for dynamic world PVP that doesn't necessarily have to be solely on PvP servers. Two forces attempt to invade the zone. One allies with the defiants in the zone, the other with the Guardians. If you are in the zone, you are asked to choose if you want to participate and if so, you are PvP flagged for the duration. The mob ratios/strength would adjust to accomodate fewer defiant or guardian forces. You'd accomplish whatever objective is set with your side and after the event would flag down (if on PvE). Maybe afterwards, you'd again defeat your former otherworldly ally or perhaps all could join in as it changes to a regular invasion event.

I'm sure other folks could think of different ideas or spins on this. It might add some occasional suprises and depth to the invasion events.