I can easily imagine a set of 'tiers' of challenges, where the bottom tier is super easy stuff for beginners and then each tier up increases the challenge until you're fighting a bunch of hard-level AI's simultaneously or whatever. Or maybe a Mario Bros-style world map that has various different challenges in different directions.
Brood War was famous for 7v1 comp stomps being very popular in custom games, and it was obviously huge competitively, it's the super hardcore competitive RTS, even if it wasn't really planned that way.
The PvE comp stomp crowd seems to really like the co-op mode in Starcraft 2 that was introduced. To the extent that they have complaints, it's mostly that they wish there was even more stuff in it.
I think for the most part, the things that competitive players and casual players want aren't really at odds. They're usually orthogonal desires.
But what does exist, are a lot of people who say "I want to play RTS, but" and will never be satisfied. Like, there's always tons of people saying that they just want an easier to play RTS than Starcraft, and there's been tons of those, and hardly anyone plays them.
Not only that. Playing a game today is going to a store, get a download code, wait half of hour for the game to install, make an online accout, confirm it, wait another half of hour for the game to update, start the game, shows you unskippable movies, then it starts some role playing and then finally the RTS or FPS starts. Sometimes you need a permanent connection to internet which makes the experience even worse.
With old games it is just: install the game then play.
The newcomers to the RTS scene are largely badly made. Company of Heroes 3 has already dropped below Company of Heroes 2 playercount despite having come out just a few months ago. AoE4 also had plenty of problems (no ranked ladder at launch? it took 6 months), though it being Relic, that's not very surprising. I'm on the r/realtimestrategy subreddit a lot, and basically every RTS that flops, well, there are very good reasons for that that you'll see people mention. It's not just "oh I'm cool with AoE/Starcraft and won't try anything else". There are some quite cool fan games, especially in the Total Annihilation-like space, but of course they don't have as much splash/marketing.
One of the core problems is probably devs streamlining away complexity, especially mechanical complexity or base building. Nearly every RTS dev for the last 15 years seems to beat on this drum of, "We're not gonna be Starcraft! We're gonna be easier for casual players!" and then they fail miserably and you look at the top 3 RTSes: Starcraft 2, Age of Empires 2, Starcraft 1. It's almost like mechanical complexity and base building can be good things!
I think Starcraft 2 had the right of it with the endless co-op mode introduced in the second expansion, that thing was very popular with more casual players (it was the most popular mode in the game at least for a while). While it retained the same core feel and mechanics of the 'regular' game, they had a lot of variety with the different commanders (basically subfactions) that could appeal to different players, including ones that were easier/had less base building. But you could still have a fairly traditional commander if you wanted one, so it was the best of both worlds.
And it's easy to see how the mode could be expanded on too: more army customizability, more variety in missions (instead of nearly every mission being 2 bases, you could vary that number up), unlockables/items/rewards from missions, varying numbers of players on a team instead of exactly 2, raid missions, mini-campaigns or storylines, etc.
Frost Giant is also planning to make 3v3 an actual competitive mode in terms of balance and design for Stormgate, which is...rare. I can't think of a competitive RTS off hand that was balanced around teams rather than 1v1, or in addition to 1v1. That might help too, team games are more social and often more popular.