Remathilis
Legend
Not to sound rude, but this is absolutely anathema to WotC's current design paradigm. First, it's a rules expansion, not a rules replacement. They want you to buy a $50 PHB and then buy $30 expansion packs, not buy a book that replaced the $50 PHB. Second, you'd need a book as large as the PHB if you plan on redoing all the classes, species, and feats in the PHB. They aren't going to do that in a 128-page book unless you cut the options down to next to nothing.It's never going to happen, but...
LOW MAGIC CAMPAIGN
Rebalanced all the species and classes to have less spells and magic.
Let us play some grounded fantasy adventures.
WotC has wisely ceded this ground to 3pp via the 5.2.x SRD. They sell a low-magic 3pp game on D&D Beyond (Middle Earth) and clearly have no interest in competing with it currently.