As a primarily WM\H player who is looking to get into AOS as a side game, I'm probably not the target audience for this thread. However, I also think I have a fairly unique perspective as such, and I think it's worth voicing, so here we go:
1.) My first act would be to bin the concept of non-campaign rules being in a physical release. New Battletomes for things like lore and art for each release will stick around, but rules will be moving to a digital format that is easier and cheaper to update and FAQ. This format will have a free to access central location in the current rules app, which will update to include an errata page. Azyr will change business model to be a one-time payment per army, and will get additional features like open war cards. One of the major benefits to this system is that it gives a lot more time for internal playtesting; there isn't the ~month of lead up required for print. It also drums up hype early, and makes sure that people who want in will be prepared to go in.
2.) Make sure that every army that is planned to be kept up-to-date is given a battletome, or atleast a semblance of one, which will of course include creating more combined factions (i.e. a combined skaven battletome.) Along with this, update the Grand Alliances for open and narrative play. Matched play for these massive groupings will be given very extensive playtesting due to the number of possible combinations and synergies, and as such will likely come out later on in the 5 year period.
3.) As for playtesting, this is probably the most radical change: Implement a system of Open Beta Testing for new models and rules. For a period of 2-3 weeks each month the releases for ~3-4 months in the future, as well as any legacy changes and general rules errata that are coming out at the same time, these rules will be available in beta form to be playtested on and reported on by community members. This would require a forum for battle reports and discussion and a specially designed form for taking feedback. To facilitate this I would look to bring specific well-known faces in the community, both from within the company and without, in to run the process. This would also include attempting to hire top players, and having them work on internal playtesting. The goal of this is to have each release be impactful on the tournament meta while also being fun to play for the casual player, and getting input from the community before finalizing the rules is a great way to do this. Also, yes it is shamelessly ripping off Privateer Press and I'm not sorry for it.
4.) Create a new system further than matched play, which is a form of tournament packet. Doing this would include trimming the scenario count down to 6 or 8, adding new scoring options and scenario\objective types (e.x objectives only scored by Flying or Warmachines, objective "Zones" instead of just points of interest, ect). Also, take a hint from places like ITC and have secondary objectives be a major part of scoring and army building. This would hopefully diversify army building while reducing the amount of work TOs need to do and increasing the complexity for the players who want that kind of tournament crunch. This change would also allow matched play to more heavily edit core game rules, like removing the rolling for first turn every turn (which I would absolutely do for this tournament packet.)
5.) Have a large narrative event with accompanying 2-3 book set every year, focusing on different areas and battles in the mortal realms with lots of fun scenarios, battalions, ect. Each of these would have an accompanying narrative league and tournament series, which together would ultimately determine the final results of the event. The narrative league sections would include a painting competition as a large part of the points scored, and both halves would have bonus points awarded for good sportsmanship.
Those would be my 5 major goals, and hopefully they sound interesting enough to get yall thinking.