I sincerely hope Intrepid is already way ahead of me on this one, but here goes.
I worry about the implications of them going from 250v250 to 500v500 battles, and their castle design. Even if they stay at 250v250, the castle we saw in Alpha 1 was not a good design for that. It's too small and cramped, and the objective was a single point at the throne in the throne room that had to be channeled at for a few minutes.
Go to
around the 1 hour 8 minute mark in their latest dev stream to get a feel for the layout:
https://www.youtube.com/watch?v=HB7gWTpeB08&t=4087s
Or check a pic I took in A1 for a better sense of the overall layout perhaps:

In A1 the defenders all respawn up behind the throne room. The attackers respawn at their base camp, and can portal out to various control points on the map, if the attackers control them. Or they can ride all the way.
Now, it's quite possible the castle in A1 was designed for 100v100. I think it was big enough for that. But it's way too small for 250v250, or worse 500v500, unless they change how the castle siege is won or lost.
If the attackers were strong enough to enter the throne room, basically everyone congregated in that single area. First the pitched chokepoint fight to get inside through the open gateway, then the fights around the pillars in there, and finally the attackers trying to hold the defenders back through the two chokepoints inside the throne room. 200 people fighting in that room is doable, but that's about the limit I think.
The two control points in the castle yard were not important enough to really fight over by either side once the throne room was open. They weren't unimportant, but it made more sense to almost all fight inside.
For 250v250 or 500v500, the castle layout has to be different. It should not be about only channeling one point for 5 minutes to win. I think they need to spread it out to 2 or 3 points at the very least. It's not enough to make the throne room bigger I think, although that would help of course. I think we need more important control points on the entire map to spread people out, and also more final objectives to win, so all 1000 people don't try to squeeze into a single room to fight.
As for node sieges, there is much we don't know yet of course, but where I can definitely see a Metropolis siege having space enough to accommodate 1000 people fighting over it, a lvl 3 node isn't much bigger than that castle, so the same issues may arise.