Much to my disappointment, as this was for me by far the biggest thing playtest-era 5e was promising: that I could easily convert my own system and in effect play 1e within 5e's framework.Something to note, the original "modularity" concept was a delusional fantasy. Our very experienced group (who all participated in the playtest) discussed it at length, and couldn't come up with a method that would work the way they claimed they could provide (characters using different rules at the same table). I think this is why the original modularity option never arose.
What was released as 5e would take so much work to convert to what I want that it's just not worth bothering.
Yes, you can add things in; but what if your goal is to subtract things out?As for an actual modular setup... it already exists. 5E is by far the easiest edition to homebrew (except maybe OD&D), where you can add a lot of cool things onto the existing chassis.
The chassis as it exists is already too complex: too many classes and sub-classes, too many options, etc. for what should be the bare-bones simpler-than-0e D&D framework.
My main problem with the DMGuild is that if I can't buy it in my FLGS (once such things return) it might as well not exist.This is where you lose me. One of the great successes of 5E has been its simplicity. Having some modular options is one thing, but it's also very easy to go too far the other way into the splatbook territory. I don't expect anything naval related again from WotC, since the necessary rules were already provided. I suspect that planar themed things will exist when/if they ever make a Manual of the Planes or Sigil campaign/AP. Same with psionics with Dark Sun (assuming they ever figure out how they want it to work). Almost everything else is likely going to be regaled to the DMGuild, which is its purpose after all.