1. Do not block armies from crossing bridges, landing, or attacking by placing heroes.

2. You may not concede or quit battle as the defeated, running down and killing lords and units is a part of the game.
Optional exception: agreed starting campaign battles to get the game moving. note some people wont want that exception hence the agreed part

3. No cornercamping/whitelinecamping.
Map terrain can be used.
 
4. During minor settlement and siege battles, the attacker must ready up first and may not move their units after hitting the ready button. This is to prevent moving chunks of armies at the last second to avoid the defender's units and defensive structure placement.

5. In simultaneous turns if an ambushing army is revealed at the start of turn, it may not re-enter ambush stance until the next turn. Beastmen may re-enter hidden encampment at the end of turn after their opponent has had ample time to react to the revealed army.

6. No retreating as the AI unless the AI is ambushed.

7. 4 max agents (heroes) in an army

8. Leadership debuffs should not exceed -25 in a given region. If unpoliced this will cause mass routes on battle map load in in the late game

9. Do not gate bug!

Specifically:

Do not sally out of any gate being attacked, or at the last moment before the gate is being attacked.

If you intend to defend a gate from being attacked, you must be out in front of it before the enemy approaches.

If units are broken and bugging a gate, the defender and attacker should cease fighting at that gate and wait for the gate to return to normal.  The attacker may ask for all fighting to stop in addition.

10. You may not uncheck the ‘Control Large Armies’ box without mutual consent from all players involved in the battle.

11. No save peeking.
Although it is nothing enforceable, for me atleast it should be mandatory.

Save peaking is the act of loading a save of a game while playing as another players faction, looking at the campaign from their POV and seeing their land, characters and lords.

12. When playing against the AI and others are spectating you must play at normal speed or above.

13. Do not change battle difficulty in game unless all players agree.

14. You can't edit saves or use any cheats/tools to adjust game mechanics in any unnatural way. I.e. it was mentioned that person had all trait from legendary lords and all skillpoint spreaded for every single skill tree for a low lvl character.

With the newest information it is somehow available. 
Punishment for that should be black listing steam account.

15. No infinite kiting/ draw kiting, attempting to run away or kite infinitely to time out is not allowed

16. When a previously determined battle must be re-fought after loading a save players must do their best to recreate the outcome.

17. For simultaneous turns, if you're army succeeds or fails in an ambush against a player, you may not reenter ambush stance until the next.

18. If you hit ready in a battle, you shouldn't move your troops.

19. When playing as an ambush on attack faction, you may not cancel an attack to reroll the ambush chance.

20. Don't do anything while a player is replacing a lord.

21. Do not enter reinforcements behind enemy held walls.

22. Do not touch the white line with your units for the purpose of blocking reinforcements.  This causes their animations to bug out and they will take damage without being able to behave normally.  You can hit them as soon as you may order an attack on them.

23. Do not use army abilities on reinforcements entering the battlefield

24. Do not move in Ambush Stance on the campaign map.

25. You may not repeat any infinite loop of actions that would result in infinite money in one turn.  Eg. getting paid for war, then paid for peace repeatedly in the same turn.

26. Mother Ostankya may not use the Malediction of Ruin against an opposing player's race.
27. You may not replace or respec a lord to take an army out of force march.