Multiple flags, possible to group them into layers that must be captured in succession. A layer could consist of one or multiple flags, and a map can have one or multiple layers.
Will be possible to set a custom round time (usually longer than what the current siege mode has). Spawning on the flags your team controls.
Also, at least one new siege machine.
edit:
Nothing is set in stone yet, needs testing, but I'll tell you how I think it will be. Attackers must capture all the flags to win the round. There will be a special scene prop that will act as a spawn point (same as entry point), each spawn point must be linked to a flag, and the team that has control of a flag can spawn on said flag (selects a random spawn point that is linked to the flag). There will be a spawn menu that you can access while dead where you can select your preferred spawn flag.
As for the flags, the map maker will have some choices. I'm adding the option to group flags into layers, where the map maker can place as many flags as he wants into a layer, and as many layers as he wants (can also place all flags into just one layer). If there are more than one layer, the attackers must capture all the flags in the first layer before they can start taking flags in the 2nd, and so on. If you add only one flag per layer, and have a lot of them, the map would be a bit tug-of-war'ish and linear for example.
The map maker (most likely), will be able to set the round time limit as well, so that's another way to balance your map. In general, I'm hoping for longer round times than we have in the current siege mode.
There will be more features in it, but I think the details above is enough to give you a rough idea of how it will play.