Yes, you can just start a thread there. The most important rules are that the proposal contains exact numbers and that you give a rationale.
The difficult part is finding the right values and making sure their combined effect isn't too weak or too strong. For unit maintenance costs, you could propose to change the values that I described earlier in this thread, and you can try these changes out in your save games to see how strong the effect would be. For building maintenance, I think the previous building maintenance proposal included a list of the costs of each building. You can propose changes to that list. As I said, balancing these values will be the hard part.