So, you can't actually set this gameplay mode in the game (can't turn off conquest), but with a little self-regulation, you can still do it.
In Adventure mode, you...
- Can only win the game with a Master Quest victory (although you can't disable Conquest victory, so you'll have to control yourself).
- Play with monster difficulty on Insane.
- Have only one AI player, set to Easy (or Hard+, but nothing in between because that actually makes it easier since you can buy from their stores).
- Cannot attack the AI player at all (you can still use their shop, though).
- Cannot have a city. You cannot settle one, and you cannot take one (not even the wildlands one).
This presents a few challenges. For one, you can't train new units. Also, the only research you'll get is from Lost Libraries and champions that have a research trait. If you play on a harder difficulty, you may be able to trade for tech as well. Also, if your sovereign dies, you lose the game.
You can play with whatever custom faction/sovereign you want.
I haven't figured up any additional rules. Despite the fact that you can't set all the game options to support this game style, you can setup the game well enough, you just have to self-enforce the rules above.
I can't remember if there's anything else to this playstyle. Suggestions are welcome. It's just a fun thing to do. I do recommend against having too many scouts. It's strategically a good idea, but it gets boring fast.