I dont think percentages are needed - if you want to max out on heroes/behemoths you should be able to. Its a theme.
Same with shooting - if shooting is too strong it should be repointed. Kairic Acolytes and Pink Horrors are well expensive for what you get - maybe other shooty stuff should be increased in points cost to match?
The scenarios themselves are also good balancing mechanisms as they ensure that a balanced army should have a good mix of troops and heroes. A scenario where only BL score could be useful as it would increase the value of the basic stuff.
Similarly more deployment zone shenanigans like in Escalation would prevent players using their optimal setup all the time and add further considerations to list-building.
None of this is very revolutionary, i think we can expect point tweaks and new scenarios as a given!
The only sticking point for me is a simple way to get a Gargant etc into an Ironjaws army (or Mutalith into Tzeentch army etc) without losing allegiance... I think maybe factions should be classified as Core and Auxiliary, so if you took models from two Core factions youd default to your basic Alliance abilities, but you could add models from one (or more) Aux factions without losing it.
Aleguzzlers, Thunderscorn, Masterclan, Deathlords, Soulblight(?), Chaos Monsters, Ironweld, Eldritch Council etc i think could all fit in this category (and more besides).
Some could be Core AND Aux (Eldritch Council, Deathlords etc).
Hmm i might try this out! Need more allegiance abilities for races, united daemons, united free peoples etc first tho to make it more worthwhile!