Some things that have occurred to me (some of this has already been mentioned):
Adjust how combat ranges work. 25 mm bases with spears shouldn't be fighting in three ranks (or two ranks with swords). This punishes low model count armies like stormcast, who are already at a disadvantage. The "simple" answer would be to adjust weapon ranges based on base size and start dipping into fractions (half inch range, 3/4 inch range, 1.5 inch range), but that might be difficult to implement in practice. They might also adjust it so all weapons have a half inch range by default but certain weapons have "reach" where they can attack through models in base to base. Not sure.
Make magic less of a zero sum game. Right now the armies who are best at casting also tend to be the armies who are best at blocking enemy magic. This makes bringing in a support wizard in a non magic focused list difficult.
Adjust how monsters work. This could be straight up warscroll changes (combat focuse monsters get higher wounds/damage) or rules bonuses (eg, monsters get +1 save, +1 damage, +1 to wound, and/or -1 to be wounded against non-hero/monsters).
Battleshock. IDK how, just change it.