So far we know it is something that will contribute to keep new content fresh, and it will effect PvP, so it is probably related to combat. The name Aegis implies that it is probably a defensive system, a way to avoid or reduce the damage you take. It is probably a bit early to start to speculate but I have a couple of theories and just wondered what others think it might be all about.
Theory one: Resistance system
Some kind of resistance system that partly resets gear progression for new zones. So in Tokyo you might need Filth resistance gear to be able to complete the dungeon and the harder missions. In the next zone this might be frost resistance and so on. It is common to reset gear progression in mmos, but this is usually done by increasing the level cap, so it is possible that Funcom might try something different but accomplish the same. The positive side of something like this is that they can add more gear progression without increasing the power of the gear, and without making old gear useless in most of the new content. Still I don't see how this could fit into PvP so it might not be anything like this.
Theory two: Extending the active dodge system
Introducing similar abilities you can slot into the Active dodge slot. Active dodge is currently the most interesting and fun defensive ability in my opinion. Expanding and improving on that system might be something that fits the few hints we have received so far. These kind of abilities are very common in recently released action based mmos, so it would make a lot of sense to modernize TSW a bit in this area and offer more options.