Slightly more refined proposal-
Collection limits (these are the limits on the collection from which your army is chosen during deployment)
Max number of war scrolls (duplicates counted separately)
- maybe a max number of 'special' (hero, war machine, monster, terrain) war scrolls
Max wounds per normal unit war scroll
max wounds per 'special' war scroll.
Maybe a maximum total number of wounds?
Deployment limits (these are limits on what you can deploy from your collection in a given game. Anything from your collection not deployed goes in reserve. Models killed during the game go to a casualty pile - available for healing effects like undead banners, but not for summoning or other effects that draw units or models from reserve)
Max number of war scrolls, slightly less than above
Max number of 'special' war scrolls, again less than above
Maybe a minimum and/or maximum total number of wounds?
EDIT: note that I wouldn't distinguish further than 'normal unit' and 'not normal unit'. For instance, I wouldn't put separate limits on war machines, heroes, and monsters, as I find that too inflexible, and poorly maps to some army themes. However, scrolls that count to the special limit in multiple ways (a scroll that is both a monster and a hero, for instance), might count twice against the limit, rather than counting once each against two different limits. That might be getting too complex to be worthwhile, though, as this system is already a bit complex due to the separate limits on collection and deployed army.
Deployment is alternating as usual, allowing you to respond you your opponent's deployment choices with tailored selections from your 'collection'.
Sudden death either not used or based on wound count. Probably best to leave the sudden death rules as scenario fodder. Actually, in 'even' games, it might be fun to give both sides a 'sudden death' condition, and use kill percentages as a tie breaker if neither or both conditions are met by the end of the game.
The collection and deployment limits would scale with game size. Individual scenarios might also affect these game rules - ie, a scenario where an advance scouting unit encounters an enemy force and must hold them off until reserves arrive might have one side starting with smaller deployment limit with more units arriving from reserve during the game, or an assault on Nurgle's garden might have nurgle daemon units returned to reserve instead of the casualty pile when destroyed, making them available to summon again and again in endless waves.
EDIT: actually, re: sudden death and the default scenario. have the collection and deployment limits scale with game size by 'tier' as per the suggestions by @Vampernata. In games of equal tiers, games last 6 turns, both sides have a 'sudden death' condition, and major victory goes to the player who achieves theirs. If both or neither player achieve their condition, then use kill percentages to determine minor victory (note that in so far as summoning allows killing percentages to exceed 100% of starting total, an army that summons multiple units may find itself suffering a minor loss even after wiping out the opposing force completely, provided they lose more models than they started with).
In games where one side is a higher tier than the other (thus having more units to deploy and more units to choose from), only the lower tier player has a sudden death victory condition, and the higher tier player cannot achieve a major victory - they can only achieve a minor victory by preventing their opponent's victory condition then winning on percentages.
In games where one side is two or more tiers lower than the other, the lower tier side gets one additional sudden death victory condition per extra tier of difference. For instance, tier IV vs. tier II, the tier II player might earn a major victory for either assassinating one of the other players heroes or for having one of their units survive the match.
This for 'normal' games, specific scenarios would interact with tiers in their own ways.