This something that can be fun. I was thinking about some of the big Charr siege weapons in The Black Citadel with wheels. How cool would it be to have a large mobile siege engine that allow multiple players to mount it at once.
One person controls movement.
One person mounts the cannon. For faster fire than catapult and good ground defense against enemy players.
One person mounts the ranged Catapult for bigger siege damage.
The mobile siege can transform into a stationary waypoint. The transformation takes 60 seconds. It's extra vulnerable during this phase. The Siege Engine runs on fuel as a timer duration before it self-destructive. This to prevent too many of them from being on the field at once for too long. 40 min fuel for the superior engines and 20 mins for the normal engines.
The Waypoint isn't contestable and is displayed to enemy factions on the map. The Waypoint on this siege engine revives in set time intervals if you want to revive here. This to be balanced for what they can do.
But once set up it can't move. The turrets can still be used.
This will lead to some stratagy usage of these and their placement will also be important.
With the increased utility that this provides the cost to make these would balance it out and act as an objective of their own for players to run supply to build it and escorts it to the location of their strategy.