I am a long time Renegade player, but still fairly new to Renegade X. Since the original, I've often thought about GDI and NOD balance. Even after thinking about it for years, really, I still am not completely sure how to address perceived balance issues.
First off, I think that there are some pretty good ideas in this post, especially in regards to vehicle balance. However, I wanted to share some general thoughts on the matter.
It seems to me that this ends up being about hard counters and soft counters as well as specific roles.
First, the specific roles. Most NOD vehicles have a really specific role that they fill and is immediately obvious what that role is. For example, mobile artillery is for sieges, stanks are for hit and run and so on. The same is not true for GDI. The medium tank is exactly that, not particularly exceptional at anything, but it also not exceptionally weak in any way either. Aside from being huge, the mammoth doesn't have a clear role either. The difference, to me, is that it's harder to do the wrong thing with the NOD vehicles (not that it doesn't happen).
This leads to people wanting hard counters. If NOD gets X, we get Y, problem solved. I personally don't like this because it (to exaggerate greatly) turns balance into rock, scissors, paper. It's less interesting than soft counters in my opinion.
That being said, I think there could be some tweaking done to make vehicles more balanced. I don't know how feasible this suggestion this is in the UDK, but if there was a way to incorporate the armor of the vehicle. Not just the health, but the physical armor. As in, tanks generally have more armor on the front than the back. So if there were a slight damage reduction to shots to the front of a tank and increased damage to shots to the back , I think that would help a lot. Maybe this could vary by tank too. For example, mammoth tanks would take little damage from frontal damage but a lot from rear damage. Again, I'm not sure how feasible this would be (or if it's already in place, but if it is I haven't noticed) but I think it would be worth at least considering.