RTS is a dying genre because its evolution has been dominated by fans of the competitive multiplayer experience of it, but that experience can now be more efficiently found in MOBAs.
Originally (like Dune, Warcraft 1) RTS was made of 2 experiences: building a base that makes an army, then fighting with that army.
I think competitive multiplayer is basically an additional experience that was improvised using the core RTS mechanics. MOBAs had their mechanics designed with this competitive experience in mind from the start so do it better.
Because comp mp was an improvisation it ended up invalidating the core experiences of building and army fighting:
>you can never spend enough time building a "cool" base that is appropriate for the visual aesthetic of the game (aesthetics are chosen because it improves the experience or fantasy) because you have to build a barebones, ultraefficient machine due to MP pressures
>you can never amass a huge, cinematic army because of MP pressures so instead all battles are small, frequent skirmishes
>you can never stop to appreciate your cool base or your cool army fighting a similarly cool looking army because of MP pressures

I think there's still an audience for these experiences, but they're currently scared off by MP and instead play Total War games, settlement games, base defense and some grand strategy.
>mechanics and AI should be tuned towards creating large, impressive settlements whilst giving the player time to do the same; creating a large impressive armies and then moving to visually impressive, large scale battles.
>replicating those campaign missions where there was a large existing enemy base and you had to build up your infrastructure to create a mega army to crack their defenses. But also expanding on it to make them more adaptable.
>more asymmetrical gameplay modes, especially if you want to include competitive MP. Ideally, MP would be co-op against an asymmetrically powerful AI.