I tend to focus on products which go against certain “maxims” of play; having alternatives to some of the more strict rules inherited from Gygax offers insight into the philosophy behind certain rules and whether such rules are actually fun at the table.
The Black Hack is my go-to book for this purpose: distances are relative, consumables are abstracted to the usage die, experience is based on stories told and not treasure dragged back to town, and all of the dice rolling can be made by players if the GM so chooses. Such a free system allows for easier hacking; You don’t need to compare relative power of classes when determining how much XP your homebrew needs in order to level up.
For supplements and splatbooks, I particularly like Wonder & Wickedness and Marvels & Malisons as magic supplements. It’s easy as hell to slap together a sorcerer class from these two books and staple it onto the Black Hack. My favourite setting books are Ultraviolet Grasslands and Hot Springs Island; both offer perfect sandbox adventures for the Black Hack. Both offer some manner of departure from the traditional tropes of TSR adventures. I have had to modify some aspects of the latter, such as the
spoiler
miscarriage statuette, which I revised as a fertility amulet.; if worn one way up you’re guaranteed fertility, the other way you’re guaranteed not to fall pregnant.
There are a stupid number of blogs and zines that I could namedrop, but the one I find myself agreeing with the most is The Alexandrian.
Ah. Are you aware of Mage: the Ascension and Mage: the Awakening? Both World of Darkness books; mechanically crunchy with a strong focus on magic as a solution to all situations. Looking at established systems that have already done something similar can help with ideas.
For a slightly different spin, I just picked up the Black Sword Hack yesterday. In terms of combat items, there are actual listed combat items but it’s all fluff really; every weapon is d6 damage. Maybe that would be another thing that interests you: weapons are abstracted to the point of players being able to buy/find “a weapon” which gives you a basic action.