I like the idea of a scenario type gamemode. It would force better teamplay and make a more intense fight than just ticket-watching like TC games end up from time to time. I never really feel a progression in any of the matches, so a slow and stead advance over enemy territory would be fun.
Just to put out some of my own ideas for this game type:
1) I like the idea to make it so bases have to be captured in order by attackers. Defenders should not be able to recapture and each team should only spawn at the bases they currently hold (ie. no spawning then getting killed by turrets as Ancient Demise wanted).
2) Attackers only recieve C-bills upon base capture (or respawn - same as usual for respawns). Defenders get C-bills normally so they can repair on the go.
3) Make the bases progressively harder to capture. More turrets and fewer angles of attack would make a base harder to take. So stick the first base in the middle of a swamp and the last one flanked by mountains with only one or two roads leading into it

4) Aircraft only spawn at each teams Main Base (the starting base for attackers and the final defensive line for the defenders).
5) Also make it so that the defenders have limited spawning options at their first base (thus forcing them to chose to spawn there as a tank or BA, or to spawn further back and sprint to get there in time to setup a defense before the attackers arrive (the distance between the first and second defensive bases should be less than the distance between the first defensive base and the attacker's spawn so that the defense still have time to reach it first, if only by a minute or so).
EDIT: Could you imagine seeing the defending mechs trying to hold back the enemy advance long enough for their artillery/tanks support to pull back to the next defensive line after a base is captured. Awesomeness!