I think it's a great idea. I don't know if I or someone else has suggested something similar, but I know I've talked about it before. I think what I suggested was just to make the Siege game mode, into a strategus type of game mode instead (for sieges). Both sides get X amount of ticks, and whichever side runs out first, loses.
People always want to see more modes in cRPG, to keep it fresh. Conquest type modes are very popular in teamwork games, and are frequently suggested, and I've read lots of good ideas by other players. You in particular always post about how the classes are mostly balanced and have their roles, but the servers have no proper teamwork so it falls apart fast. That's why a mode that encouraged and rewarded classes filling their niches on a battlefield would probably just make it a better game.
Something that I always remember from reading WW2 soldier's memoirs is just how expendable the rifleman was treated. In cRPG, the melee infantry are usually the riflemen, there to throw themselves at each other, all while being torn apart by ranged and cavalry support, to fight over ground for the greater good of the team. Nowadays, there is no greater good than points, and you don't get points by actually taking that god damn hill full of archers away from the enemy team. You get shot from 4 different directions, then die hopeless and alone, hoping some str builds and lance cav left alive can track down 5-10 kiting ranged or horse xbows for a multi that sometimes doesn't even feel worth the bother. This is very typical Battle server gameplay, and it probably turns many new players away, and burns veteran players out faster than need be.
Battle and Siege can be plenty fun, but that is more due to the really solid Warband game mechanics then the modes themselves. One is a counter-strike clone, the other is.. well, Siege. I just see a lot of wasted potential in all the new things the devs have done, such as Strategus, which could be integrated with existing elements of the other modes to create something that would really play to this game's strengths and be a real reason to play it over Native or any other mod.