Having groups of different optional feats categorized by "theme" would be cool. Ravenloft has a group of setting-specific rules (Survivors, Stress, cosmetic spell changes, etc) that could replace the Horror and Insanity optional rules from the DMG. They could do similar things for the Gritty/Survival, Nonlethal, Tactical, and other types of play in the game, probably even calling out settings and campaign ideas where they'd work best (Horror for Ravenloft, Gritty for Dark Sun, Nonlethal for the Feywild, Tactical for Dragonlance, etc). Good idea!So a few thoughts.
First, I think the use of the DMG for optional rules is the best use possible. It is supposed to fire the imagination. For those who choose to engage with it, the book is supposed to give you ideas and options. Not all of them are going to things a DM is going to want for their table, but some of them will be the seeds for how a table is going to adjust the game to make it play the way they want. It's fully in keeping with the character and the history of D&D that a primary purpose of this rulebook is to remind you that the table, not the book, is in charge of the rules.
That said, I think the flanking (and other miniature rules) are both a decent start but also under-developed. I'd prefer that the DMG, instead of having it either more developed or less developed, instead have a description of the differences between grid/minis/TOTM play, and have more advanced mini play as a separate supplement. Flanking is good if you like minis, but people that are really into the combat modifiers and minis are going to want a lot more.
What I think might be interesting is having a more built-out and "themed section" with grouped-together optional rules ... as if they were various levers. For example, if you want to ratchet up the resource management and grittiness, there's a bunch of different optional rules ... why not have them together in one section, as different aspects? Or if you prefer a more narrative approach (or more player control over fiction) have those ideas together?