Having a bridge that is linked to a base somehow and that can change hands without the base itself changing hands isn't really possible the way the game is currently written. If the bridge has any sort of map room (and it would really have to be just one map room with 10 troops) it is effectively its own base and would appear on the clipboard map as such. This lets players know who owns it, it flashes when enemies are near and so on.
So I think for a capturable bridge to work without a major recode of AH it would need to be a new type of what HTC call a "square tile object", like a vehicle base or airfield etc. This could have a bridge crossing a quite narrow stretch of water that would widen out to each side of the bridge. The water would need to widen out as there is a limit as to how narrow a coastline river can be made in the game before both shores merge into one. The problem is this makes for quite a wide river, more like the Mississippi than the little supply route rivers AH bridges are designed for. So our bridge base shore lines would be designed to pick up the narrowest possible terrain coast line river and either funnel it down to a narrow enough gap for a reasonable length bridge, or perhaps two bridges with an island in the middle like this:-
This bridge base would have no vehicle hangar or any SPs radiating away from it. The idea is the terrain maker would place the bridge base close to two small V bases, one on each side of the river, and the players from each side would drive from these V bases to the bridge and fight. So capturing the bridge base would be pointless unless one of the V bases was taken first and capturing the bridge only gets you access to the other V base. Capturing that V base then gets you access to SPs going to other bases. Hills overlooking the base on each side which would let players fire over the island to the opposite bank (which might give the King Tiger or Jagdpanther more of a role in the game).
The island itself would have just a map room, some revetments, plus some player-controlled AA and AT guns for defence. Having no AI guns lets the bridge base be placed closer to the V bases, this is because its an MA rule that no two bases (or an SP) can be placed within range of enemy AI guns. The bridges could be destroyed by whoever does not own the field in order to prevent an enemy advancing across the river but it would take a lot of ordnance to do and a lot of supplies to restore.
I think the above could be done using existing game objects and without any need to recode the game. It would require the bridge base to be created but I think this could be done by players familiar with doing similar stuff for special event terrains rather than Hitech having to hire an artist or 3D modeller. In general though I have found HTC are very wary of anything "interesting" being done to an MA terrain. I learnt this the hard way when making CraterMA, I had to erase months of work a couple of times due to me being a little too creative with the way I used certain objects in the game. This is because an MA terrain must be absolutely stable 24/7 and never ever crash, far more so than an event terrain. So if any such bridge base were created it would have to be extensively tested before it went live.
