Siege is the most fun game type, but I think it may have too high a learning curve for new players. Although siege has the highest points yield of any game mode, teams with newer players get crushed and earn a pittance for trying to complete a game compared to the team with experienced players. A co-op siege mode would give newer players a chance to cut their teeth on this fun and engaging game mode without the despair of a long, fruitless game.
Here's hoping to see more players on siege servers!