My guess would be the same vertices resolution as now. HT has said the current engine supports double this resolution now but it isn't used as it would screw FPS on lesser PCs. Unlike the new high res textures, increased vertices couldn't be turned off by players with older PCs.
I agree it would be great if HTC could take this opportunity to add some new stuff for MA terrains: bridges or bridge bases, forward airfields with no ordnance or even no hangars, just a reload pad. Maybe an AT bunker with a 17 pdr that could be added to any field or town's defences by the map designer in a similar way to a SB. Or just some clutter objects like ruined factories, wrecked ships and the like that could be dotted around the map to add visual interest.