Don't get me wrong. I would rather play with all the 3K toons but once I tried to play the game as a standalone game, the campaign is a lot more refreshing than the base 3K. I may have burnt myself out on the 3K campaign but as a whole, here are some major things that standout to me:
1) Factions feel a lot more unique. Rather than "You're another dude with a diff set of currency", I had to adjust my playstyle rather than standard "expand fast, build more armies" especially considering ...
2) Jin Empire isn't just free land grab. If you plan on warring these guys, you better be planning for the long haul. Not sure if you can keep getting edicts (which are incredibly helpful) from them if you're at war but they feel a lot more fleshed out than Han Empire as they keep standing troops that actually fight you but also introduces nice mechanics to keep favorably with them. Regency vs Emperor and their diplomatic repurcussions is way more flavorful than "Han Empire is now your vassal so you're perma war everyone else??" (which feels really stupid - why is controlling the puppet emperor just a gigantic detriment in base game??)
3) Minor balance improvements. Schools add some PO rather so it's worth considering rather than a pure "build this to get the archer reform and blow it up after". There's also a building that increases research speed which is great as reforms really just go down the same two trees and then the game's over. I'm also not sure of what happened but it seems I've been really sparse on getting Champion and Vanguards. Combined with the nerf to turtle, I've actually been running diff teams at times such as pure infantry + archery mix just because of the available generals and retinue so it's added some nice variety to battles for me.
4) Alignment is a really nice extra touch as opposed to yet another "your generals get into some argument about a pot and you decide who give the pot to for a temp satisfaction ..."
5) "Grand missions". Control 12 commanderies for +3 perma admin & prestige? Build 80 units for another perma bonus? These felt awesome to accomplish and the rewards were huge. I love how they commit you down the alignment path and give a huge boost.
6) Faction Leaders feel REALLY powerful. (They get so many stats, empire perks but they also get their own customized tree) Added just a nice extra individual touch.
7) The victory condition makes more sense, has more pathways to attaining it, and feels a lot less grindy. You can even choose to go down the "ultimate victory" condition if you want to do color the map.
8) Research feels more powerful due to the slimmed down options. I actually feel like I'm getting something each step rather than just waiting till the end of a branch or unlocking some basic buildings/units.
9) Game feels more "slowed down", see Jin Empire notes, but I definitely find myself using the diplo option. At times buying land versus just conquering to keep the relations.
Honestly, if we could pay a few extra dollars literally to just port these 8P mechanics to 3K base, I think I could probably blow another 200-300 hours onto 3K base easily.