I'm one of those people who thinks SC2 is far and away the best game Blizzard has ever made (mission design in particular has only been on the up and up in terms of variety and mechanical interest ever since WC3: TFT), but it's true that since WC3 there has been this divergence between StarCraft's identity revolving around huge economies and large, disposable armies, and WC3's identity revolving around tightly capped economies and small but beefy armies, with the common factor in the Blizzard brand being the radical factional asymmetry. Obviously the WC3 style has been MIA for a long time now and much of its appeal has been subsumed into the MOBA space, although I'd like to see it back in some form as I think there's still a space for it.
Moving towards smaller play sessions, be it in shorter campaign missions or multiplayer skirmishes, seems antithetical to expanding RTS to make it casual-friendly again, though. The bar for most players that discourages them from RTS is the speed. Novice or low-level players are the ones who always prefer to turtle up slowly, play it safe, and tinker with their bases without getting too stressed about resource depletion or the action elsewhere on the map. This is why tower defence (on the all-macro end, with MOBA at the all-micro end) took off the way it did back in the WC3 era: it was slow enough that it was practically turn-based, while leaving considerable room for mastery or creative teamwork if the TD map was well-designed.
One major space in RTS that Blizzard has never touched, and which could be ripe for an explosion given the right game, is the kind of real-time equivalent of 4X strategy that one sees in games like Sins of a Solar Empire or the space layer of Empire at Wara macro-centric game that is about economic decisions and build/tech pathways, but at a considerably accelerated pace (think of how Rise of Nations felt when it first launched, as a Stone-Age-to-nukes experience of territorial control that was like condensing Civilization into an hour). That is territory open for the taking.