This is prone to a whole host of problems, not least of which is Ship of Theseus syndrome. People have enough trouble confusing various editions of D&D with their small to large differences. You go changing the periphery of the root game (whatever that is), people are going to have more confusion rather than less compared to systems they know from the get-go are distinct (although as we've seen time and again there's always baggage moving from system to system).
I and others in our crew have over the years pretty much rewritten 1e D&D from the ground up; and yet I think someone who has only played 1e by the book would still recognize it not only as D&D but as a version of D&D that bore considerable similarity to what said player was used to; and that it would be easier for that player to adjust to our system than it would be for, say, someone who had only played PbtA games or even 4e D&D.
It's also a lot of work to kitbash. If you want an original magic system, you have to come up with it pretty much from scratch, except it has to work with the existing mechanisms of hit points, saving throws, etc. etc. Or is D&D's magic system part of the root game? If so, then D&D will never, ever fit my set of preferences.
It's a lot of work for the GM, yes; but almost none for the players.
What are your preferences for a magic system?
If you want to have formalized faction relationships, you have to make that up.
To me that would be part of worldbuilding or setting design, similar to making up kingdoms and guilds and so forth and figuring out how they relate to/with each other.
If you want to do combat in any way other than round-by-round, turn-by-turn, hp-ablative hit-or-whiff mechanics...well I guess that is clearly part of the root game, so I guess that's out of consideration.
More or less. Hit or miss? Yeah, that stays. Round-by-round turn-by-turn? Well, sort of - round-by-round kinda has to stay if only as a timing mechanism (though one could, I suppose, go to minutes and seconds), and turn-by-turn can be changed up by allowing simultaniety and re-rolling initiatives every round. Lipstick on a pig? Maybe; but still better than nothing.
If you want to go even broader and do conflict rather than task resolution, or actions with complications/consequences rather than success/whiff, you have to mutate the core d20 mechanic almost beyond recognition.
On this I disagree, in that if instead of looking at a roll as a binary pass-fail you look at it as also informing the degree of pass-fail it's easy to introduce either or both of fail-forward or success-with-complications on rolls that are close to the succeed-fail cutoff.
If you want characters that aren't defined by strict classes and levels, again, is that part of the root game or not? What even constitutes the root game of 5e?
Yes, those are all part of the root core.
Now, there is a lot in 5e that can be tailored and customized to fit different allocations of authority in setting & narration, genre trappings, and the like, but the root game (such as I'd expect many to agree on) remains one of success/whiff task resolution, a heavy mechanical focus on round-by-round ablative combat, and pretty fixed classes...whose abilities are heavily focused on round-by-round ablative combat and whose advancement is pretty regimented. And there's a big, wide world of other game styles than that which no amount of kitbashing around the periphery of that root will get you to.
I think at an even more "root" level the one thing I'd want to keep as a guiding principle is the ability to get down to a very high granularity of task/conflict resolution - i.e. resolve things step by step rather than all in one go, even if it takes longer at the table. This is where I fall off the 4e skill-challenge wagon, for example.
Enforcing speed/efficiency of story advancement isn't even on my list of priorities; it'll advance as fast or as slowly as the players and GM want it to and if it's slow then so be it. There's always another session.
The rules kind of have to pay a lot of attention to combat as that's the one thing that pretty much has to be done entirely in the abstract. That said, round-by-round ablative combat has pros and cons to it, no question there; perhaps the biggest pro being that IME players just love-love-love! rolling dice and combat gives lots of reasons to do so.
