This may be controversial, but as someone who doesn't like AoS but sees the GH as a step in the right direction and with some more developement could become a game I could play here is what I would like to see in no particular order:
1) More custamization, mostly of characters. More magic items and full spell lores for each faction as per the Sylvaneth book.
2) Modify the rule of one with regards to spells. Multiple instances of the same spells don't stack, but the same spell can be cast more than once. This may not be necessary if everyone has multiple spells than can cast. Would like to see wizards play a larger role/ multiple wizards be a viable strategy.
3) Measuring from base to base. Would also like to see some base standardization, or atleast what is "'officially recamended"'.
4) Battleline units may not be necessary with the rules structure and points as they are, other games manage it.
5) Clarification of unit options. Explicitly state what each unit comes with and how many of each option it can take/ which are mutually exclusive. I find that this isn't alway clear.
6) Points to be included in the warscroll, this can then be updated on the fly to balance things if necessary. Navigating between the scrolls and the GH, while not much of an inconvinience is still a bit of a pain. I beleive that the GH has enough additional content to not invalidate it.
7) More rules/ restrictions governing movement. Difficult terrain/ pathfinder etc.
That's it off the top of my head. These may be seen as too many core changes but the GH amends the core rules for matched play so I dont see this being a problem.
Thanks.