I'm not sure why I stopped playing seige. I felt the monotony after a while that I don't feel with battle mode. The reason I continue not to play it is because it's not as fun for me, and you get switched every round. Some core mechanics probably needs to be changed for me to go back to it. There's no more situations where I feel like "I guess I'll play siege," I just completely exit out of the game. New, interesting maps and good spawns are definitely a start. Being able to utilize multiple facets of the map makes it interesting.
While I will agree that getting switched almost every round (from winner to probable loser especially) does get tiring, it is still far better than playing defense 10 rounds in a row. I really don't know if there is a single player who prefers defense over offense. It just has so much more downtime.. sure, it takes about the same time to get back into the fight after dying, but at least as an attacker you spend 30 seconds running around, planning your attack, and not just watching what others are up to, praying that you will spawn in a good spot.
A selectable spawn option for defense would shake things up a bit... nothing quite as bad as spawning on the walls when the attackers are capping the flag. Although I am sure that if everyone just chose one close to the flag, it would get a little tricky to cap the flag. I just think that if while spectating you see maybe 5 guys on the backdoor, then you spawn in the front of the castle where you can't do anything except type "incoming backdoor" makes you feel so useless. Whats the point of spectating if you can't directly act on it? It would be great if you saw guys going to the left side, so you switch your spawn from right to left, and as soon as you spawn in you get to put down their little sneak attack on your own (or die trying, no doubt gloriously). Maybe having 4 general spawn areas could work? Back, front, left and right or something to that effect.