I have played both TW expansions. A lot.

I enjoyed the new features in each of them although IMO none of them represented an especially dramatic departure from the game play of its predecessor. The Mongols don’t build troops and that was the big difference from other factions. A new map and new factions in VI were excellent but the other changes were minor. The reinforcement tweak was nice but was hardly a sea change in game mechanics.

Night battles and river crossings sound like significant tactical additions to me but deciding what are major and minor features is a very subjective undertaking. One man’s delicacy is another’s gruel. The migrating barbarians in BI could be very interesting and different. I’m not sure we know what all the next expansion will include.