There is no doubt that Magic is a commercial success. That is, it has mass appeal. And in order to appeal to the masses, it has to sacrifice and restrict certain things. Like new strategies, new deck archetypes, new mechanics that redefine how Magic is played (for instance, Planeswalkers added a new dimension to board state and deckbuilding, something that hasn’t been seen again since it was introduced in 2007.) And as Wizards continues towards this trend to cater to new players and limited players, less and less set space and development time will be allotted for actual fresh, creative gameplay. Hence even Wizards R&D managers admit that Magic design space is becoming more and more limited. However, this is of their own doing due to how they design cards and sets. Even though ideas are infinite, they restrict themselves to a tiny subset of possible ideas in order to sell more cards, not necessarily making the best designed cards or opening new gameplay.
Not all design space is equal. I mean quantity vs quality. Sure they can push out 1000 cards a year. But in reality the vast majority are rehashing and tweaking old cards. How many actually change the metagame? How many make you think or play differently, or enable totally new strategies? How much are actually never-before-seen abilities?
Look at Theros and tell me how much new ground was actually treaded. Devotion is redesigned as chroma from Shadowmoor. Bestow is living weapons meets licids (BTW bestow are mostly uncompetitive even though it had so much potential to spawn new deck archetypes. Again catering to limited actually hurt a card type they were supposed to promote.) Tribute is like punisher from Odyssey. Scry is directly reused from Fifth Dawn and Magic 2011 core set. In short, Theros has been the low-point of design in ten years or so. Not just because they focus too much on creatures and limited format, but also because they played it too safe (a trend that started since Return to Ravnica, right after pushing the limits in Innistrad.)
As a whole Theros contributed very little to non rotating formats. First they are playing too safe with new cards. 90% are aimed for limited, which means underpowered and dumbed down. So 90% of a new set do not push new design space to its potential. Second, the focus on creatures means they are constricting design space for other card types and narrowing the possible strategies for the game as a whole. As such, the metagame became predictable and repetitive. Many herald Return to Ravnica/Theros standard as one of the most boring, uncreative Standard season ever. Right now, the same 3 archetypes dominated for 9 straight months. Even compared to Kamigawa, another weak set from a decade prior, at least Kamigawa brought quite a few powerful spells that redefined Legacy and Extended, which is now Modern. Unfortunately, Theros has no such distinguishing cards. This shows that constructed formats will remain stale as long as they stop pushing the boundaries of design space. Why would one buy new cards if they don’t actually do anything new?
As long Magic remains as a creature-centric game rather than embracing all types of noncreatures strategies (in particular, combo) which are supposedly more complicate and unfun, its other major flaws will be exposed. In particular the range of design space seemingly is becoming narrower and narrower, hence they try to stretch out the mechanics over many sets and blocks by intentionally weakening 90% of the cards. So yes, they have ideas for new sets for many years to come. Unfortunately, be aware that the vast majority of it is boring filler. The other major problem, which I’m not sure if they’re aware of or not, is the strategically inept combat system. In its current form, Magic cannot possibly encompass anything more than shallow creature on creature combat. That is, since they are almost totally focused on creature based interaction, however, their combat system cannot provide deep, meaningful, interesting decisions and interactions compared to other CCGs that are built with more intricate and deeper combat systems. Now if they revamp the combat system, then their design space would widen quite a bit, at least as far as creatures are concerned. It still leaves a wide ocean of design space that is very unlikely to be addressed in the near future.