Not sure if you are looking for advice or just musing out loud. I think depends on are your potential changes additive or subtractive.
My advice would be to go in the direction with the least amount of subtraction.
It will be interesting to see if we can get some wierd going. I have not looked past core D&D, since I try not to buy if I will not get to run it but some of the third party might be exploring the weird.
One of the interesting things about third party D&D is to be compatible they are limited to...
At a continent or world scale, the short answer, is I would not. Just too much work and very poor granularity. With Ebberon they have magi-tech at about late nineteenth century and just ended a world war. There are probably good maps and gazetteers for most places.
Get your players to tell you...
Yes, to the posturing, with the odd random death when some misjudged thier distance. And it would only last until someone traineed their people to gwt stuck in.
You say that predictibality of D&D combat is unrealistic. Ans you are right, but I suspect that is why it gets used and played. Most...
We did not get a new edition, because calling it a new edition would kill sales in the back catalogue. The differences between the two version are not enough to make the older books useless. Most are still useful, particularly the adventures, setting and lore books.
Tasha's and Xanathar's are...
Much less of a problem, though perhaps this is more of a problem in the internet age than before. Before internet one when into a shop and bought the current books on offer.
D&DBeyond is something similar, there is (I presume ) for a new customer no sign of the 2014 core books.
As for the...
Most people that play D&D and have never seen real combat, movies are often the reference point for combat. Now of the people that want "realistic" combat, they actually want cinematic combat when you drill into it. Some just want gritty, that is dangerous combat.
Too much interconnected lore becomes a barrier to entry at some point. It happened to me with D&D back in the day. Walking into a store an seeing all the books, and thinking "I have to buy and read all that?" I had strong completionist tendencies back those days. But I also know of people that...