As I think I might have said before, for sentimental reasons 2nd edition AD&D will always be my favourite version of D&D, but I'd always make the case that the best is BECMI. ('Favourite' and 'best' don't always have to be the same thing, I'm sure you'd agree. My favourite TV series, for example, is Star Trek: The Next Generation, which is hardly the best ever made.) Why BECMI? Because it perfectly melds the make-it-up-yourself aspect of OD&D with a more comprehensive and sensible framework of rules - moreover a set of rules which work as one volume and don't become overly complicated or require you to memorise very much. No edition of (A)D&D ever managed anything like that.
So I think on the face of it I'd like to make my Mollusc World setting a BECMI/RC one. But this could cause a problem.
Viz: just how popular is BECMI these days? The so-called 'legacy gaming' trend tends to focus more on OD&D and 1st edition AD&D than anything else. As far as I can tell BECMI comes a distant third, only just in front of 2nd edition (the big loser in the 'old school renaissance'). Labyrinth Lord is partly to blame for this, in that it scratches the itch that Classic D&D used to. My suspicion is that the people who would have been using the Rules Cyclopedia to run games are now mostly relying on LL. (No bad thing in itself, but I've come to the decision that for aesthetic reasons if nothing else BECMI beats LL hands down.) And there is still, I think, an entirely misplaced prejudice and dislike of race-as-class at large in the D&D playing community.
Just how much interest there would be in Classic D&D products these days is questionable.