Without stating what everyone else has stated:
*Have XP be more than just multiplier tics. I fear multipliers with flag captures where you go back to 1 multi if someone caps your flag, could be too severe and could force people to have 1x multipliers for a rather extensive amount of time. I can't stress this one enough, especially due to balance not switching up teams as often as it does in other game modes.
*~30/40 minutes per map would be nice.
*Focus on large maps rather than small ones.
*No village maps. Village maps make for especially shitty CTF maps. Maps are going to have the same amount of importance in CTF that they have in Siege, if not more so due to CTF simply being a more complex gamemode.
*Don't allow cavalry to be battalion flag carriers. A clan of 15/20 people could ensure victory nearly every round by having only one guy spawn which a horse, get a battalion flag, ride to one of the flanks near the enemy flag and then have the rest of the clan spawn on him.
*Don't allow spawning on a teammate who is within 20 feet of the opponent team's original flag location.