I think Titan works well with the objectives above the land map. But for BF4 I think there will be a map where attackers go for objectives on a carrier, and both sides will have subs which will act like replacements for the AMTRACs in BF3. Subs could be used as transport up to 6-8 soldiers to the underbelly of the carrier and attack from below...of course the defenders would have to search for the sub and make sure it doesnt get into a strategic position, amphibious assault vehicles would have to counter the sub, but the opposing sub would be best to destroy it...and i think the sub cant be spotted only can be seen by radar of driver of opposing sub or amphibious vehicle. Something like that i think.
May need a BF4 thread on forums.