A simple solution would be to make a conquest game mode allowing up to 8 players to join. Myself and our group currently have 6 players, meaning 2 miss out. Have the current set up for conquest alongside a designated mode designed to cater for larger teams (8 players split into 2 squads of 4).
We're constantly reminded battlefield is a team based game whilst being limited within that structure. Just create some game modes designed for larger groups. That way you keep the core of the game modes for those without large groups whilst distributing the larger groups elsewhere
