With the understanding that I'm opposed to the entire prospect well established, if you do go forward with this, things I'd like to see include....
I'd like to see several of the rules that encourage huge units in 8th edition mitigated or just rolled back entirely. 15 to 25 should be respectable unit sizes, imo, there shouldn't be this painful upward pressure pushing people to run unwieldy, overpriced, and boring units of 40+, which you get from having steadfast and hordes in the game.
Also, go back to loose formation skirmishers that rank up only when locked in combat, or else just have the unit rank up in base contact normally all the time and have skirmishing just be special rules rather than a physical formation. The half-arsed, rectangular formation only spread an inch apart so it's crazy awkward to move them around and they still have to squish together in close combat so you can't even just put them on a spread out unit tray business is terrible. Terrible to play with. No good. I know skirmishers dancing around, always an inch from a unit but never in its charge arc were annoying, and that's a problem for fliers and fast cav too, but just fix that problem directly, by letting units WALK into close combat. IF someone is so close to you that they can give up half their move to turn (or just quick reform or whatever in 8th) and just walk their normal move into close combat, then they shouldn't have to charge you to do so. I mean, obviously they'd get no charge bonuses, but still.
Stopping in the middle of the game every time you touch terrain to roll what sort of wacky thing it does and then having to remember that for the rest of the game is terrible. Also, terrain that does every ridiculous, awkward, gamey thing under the sun EXCEPT the ONE THING that terrain is intuitively supposed to do - slow down units that move through it - is terrible. Let a forest just BE a forest. Maybe difficult terrain slowed units down too much before, maybe 'half movement' OR 'no marching' is appropriate, but not both? I don't know. But come on, here, seriously. 8th ed terrain is a mess, scrap it completely and go back to 6th or 7th, then improve from there.
Magic dice pool that doesn't scale with game size is a problem. Magic dice pool that scales with how many magic levels you field making the magic phase heavily manipulable and thus super abusable, and punishes any players who dare not go magic heavy is also a problem. Magic and dispel dice pool should scale with the size of game directly, not 'not at all' and not indirectly by what players field. How you fluff it doesn't matter (I like bigger game -> more soldiers -> more lives in the balance -> more magical power drawn on to the battle by the winds of fate), but that's what it needs to do somehow to actually work. Also, some super spells are just too super. Maybe we could just, you know, 'not', with the purple sun and pit of shades and mind razor and all.
Get rid of true line of sight. It's a game play abomination. No minis game should ever use true line of sight. Ever.
Consider complete reworkings of major parts of the rules. I mean, if you're coming up with your own version, why not, right? Try things GW never did. Question everything. Or rather, question things I tell you to question. Or rather, question specifically the 'I do everything while you wait around, then you do everything while I wait around' turn order. Mix things up. Go lord of the rings on this, maybe, with one player moving, then the other, then first player shoots and then the other, and so on. Maybe have combined phases where you take turns unit by unit. Maybe have a singular magic phase where instead of one wizard casting and another dispelling, two wizards are nominated to attempt to wrest control of the winds of magic and whichever succeeds gets to cast a spell, repeating the process until the magic is used up for the round or all the wizards are exhausted?