The question I have is: Is it easier to start with the "hyper" assumption and pare it down for the "mundane" campaigns, or is it easier to start with "mundane" and add on modules for the "super"? What should the Basic/Core game presume about Fighters and Thieves at low-mid level?
I really dislike both the terms "mundane" and "super" in this context... It really does irritate me how people equate "not strictly realistic" with comic book superheroes (an association with all kinds of messy implications and assumptions that mischaracterize other's preferences). It also irritates me that anything in a fantasy game like D&D be bound by the word "mundane." Even a non magical character is a character of fantasy, and shouldn't be as
boring as the word "mundane" implies.
My annoyances with your phrasing aside, I wouldn't really break things up into merely low-powered and high-powered, with some assumption that low-powered is less complex and high-powered is more complex. Rather, I'd say the baseline is somewhere between, in the realm of typical fantasy assumptions. "Gritty" play where characters start off as weaklings and anyone can die at any time would like be a more complex rules addition. "High-powered" play where characters are capable of impressive feats and can cheat death is likely a step more complicated than normal. At the same time, a concept of "high-powered gritty", where everyone is more powerful but more at risk of death at any time, is a feasible combination of the two. They would both be modules that add on to the more simple baseline.
I also disagree with an assumption you make earlier in your post, about how "epic" play and "castles and kings" play are diverging paths for the game. They are certainly both options for high-level play, but they are not incompatible. A character who can cut the sky with his sword can still be a leader and king, and that option of combining both options should be valid.
Romance of the Three Kingdoms style play should an option.
Overall, I think the important point is to not think in binary terms of "one or the other" for this kind of thing, and to certainly not try to put your preference forward as the natural default and someone else's preference as the "option". Generally, such a system works a lot better if the middle ground is the default.
I hate to be that guy--and I don't mean to be rude by suggesting it--but there are other game systems that cater to anime/wuxia-style fantasy superpowers play. Exalted leaps immediately to mind. Maybe the people who want that sort of thing in D&D could go play that game instead...
If you know you're being "that guy" and that it might be rude, why are you suggesting this?
It really
is rude to tell people that they are not "real" fans of D&D and that they should go off and play some other game so you don't have to put up with them and their preferences. After all, the idea that one playstyle (that D&D never fully embraced!) is "true D&D" and that the other playstyle (that is seen often in D&D!) is "not supported properly" is a very flawed perspective. D&D has always tried to play to both sides and be a game for everyone. 5E's goal of trying to quite literally provide an option for every playstyle is just a natural extension of that. I see no reason to abandon D&D for another game when it works just fine for me 90% of the time. I certainly have no desire to switch to a totally different system like Exalted with its own set of annoying preconceptions when D&D
can work just fine!
D&D only fails to work for anime/wuxia fans in the minds of people who don't like anime/wuxia fans enjoying D&D.