Okay, sat down for a bit and made some images to demonstrate potential things I'm thinking of doing. Let me know what you think.
First up, a potential change that expands the base area such that the team vehicles are contained within the base rather than outside of it (and thus vulnerable):

Placement and whatnot are subject to change (most obviously, I'd move the Hellbender inside), but I'm hoping this change will improve defenders' odds considerably.
Here's another idea. Pretty basic:

Add a side route to that one particular node. I think adding a side route would make it considerably easier to attack that node and improve the endgame dynamic a lot.
A corollary would be to add a new node (indicated by the X). This could either be a spur node or a more easily defended / not-uphill primary node.
And, well, this next image is kind of a grab-bag of concepts:

-The orange arrow here would indicate an underground tunnel that small vehicles and footsoldiers may be able to pass through. The idea here being to add another route to bypass the wall in the center for ground-based vehicles. This would probably be a territorial advantage to whoever held the middle node.
-Green Xs indicate potential positions for additional nodes, one being atop(/inside?) the towers near the middle, the other being atop the terrain near the core (and most likely only accessible via flyer or teleporter).
The towers strike me as an ideal location for a spur node that would allow a losing team to get men back onto the playing field from a deep-strike position. However, I'm not sure that's a good idea since it'd be pretty irritating to have guys camping up on those towers with Avrils all the time. -- Still, the node might be inside the tower, possibly add a teleporter to the top...
Having a node up on the cliffs near the steam primary is sort of a variation on adding an actual route to that node. As either a new primary or a spur node, it'd make it more difficult to keep the losing team pinned down.