The problem is that the gameplay has become stale and frankly, dated in many aspects. Assault Horizon was the attempt to fix this, but everything new they added was worse and everything that could have been revamped was left alone. They copied concepts from infantry/personal based games and implemented it into a flight game. The result was disastrous. I still play AC4/Z and a bit of 5 every now and then, but it is hard to do make a unique gameplay experience if they do not expand upon characteristics inherent to military aviation. Gameplay, if altered, must be done in a way which fits within the theme and technical aspects of military aircraft.
I've brought up ideas before, even posted them here. Remove the universal all targetting "missile" with dedicated weapons. Have a large number of weapons, even loadout customization. All of a sudden that F-15C becomes near useless for A2G and that A-10 can carry an excellent variety of A2G weapons all at once. They start to fill their own niche. Different weapons for different tasks: HARMs for SAMs and ground radars, anti tank missiles (quicker reload time) for lower damage. Anti ship missiles for larger amounts of damage, longer range, long reload time. Different weapons for different targets. Many possibilities.
Make enemy ground radars and AWACS useful; if they're active they can vector in enemy fighters from further distances making life a pain for players. It makes hitting high value targets more worthwhile than gaining some extra points.
And for multiplayer give realistic loadouts. 60-70 missiles for MP just turns into a missile spamming contest.
These are just some ideas I'd like to see in a future Ace Combat or similar game. I'll still get AC7 regardless, but even I am getting a bit tired of the same core gameplay which has been unchanged since 2001. I think the problem is the average gamer is too stupid to take the time to learn about the subject, and having to figure out what different types of weapons work best for what will be a roadblock to new players. People put a lot of time into learning game mechanics if the subject matter is well covered, such as fantasy RPGs or 1st/3rd person shooters but they do not like to be challenged by new concepts.