I think Barbarian is fine, but a -10% XP penalty on Tactician might be a good tweak. I find it harder to motivate myself when I reach max level.
If you could go "gestalt" after 12th Level, that would help (like each Level allows you to gain the benefits of 1 Level of any Class, but only improve hp or increase proficiencies if the "overlap" results in a better result). It would certainly be good enough to keep my interest and it would be fun to game out the benefits of different classes, but without increasing Proficiency Bonus and increasing hp dramatically, it would still prevent you from being too OP. I'm pretty sure there isn't enough XP in the game to get more than 2 or 3 gestalt levels anyway. So you would feel like you are advancing all the time but it wouldn't break anything.
