The following suggestions are based around the core idea of fun (i.e. what I don't find fun in AoS):
Reconsider battleline and how it is implemented by default. Why is being forced to take units I don't want to fun? Why is it fun to lose out on a bonus for XX number of models due to trying to hit a predefined limit of units?
Spells/abilities per faction. They'll come, but please hurry it up.
Rule of one on spells is reactionary to an open play issue. Massively impacts some factions (Tzeentch) whilst others get around it with loopholes (keyword changes ((Stormcast)). If you're then writing rules to get around your own rules - i.e. Tzeentch suddenly cast changes rather than spells - it doesn't fix the problem (the original rule). And at least change to successful attempts.
Avoid at all costs using different keywords to get around your own rules. 40k has rules stacked on rules just so one army seems a bit cooler at the expense of rule bloat and the other player being annoyed (i.e. it's not fun).
Vary the missions. Hold the objective is over used.
Keep it simple as they have done so far!
Allow all chaos monsters to have marks of chaos to get around allegiance issues (like the Vortex Beast, for example).
Rather than increase any point costs, instead look at the units no one uses and ask why. Are those too expensive? Do they need an ability/combo tweak? Don't immediately punish the good units. Improve the bad ones.
On top of that, it'd be neat if GW conducted usability testing at events. That can give you real insight as to what the pain points actually are. Asking directly what to improve, from a product perspective, isn't always wise.