Is this sort of thing something you would be interested in? If so, what would you like to see in it?
Yes in general, with a big "BUT" coming...
- The fighter class reprinted for convenience
I'm actually okay with this; even if some stuff is reprinted, having everything in one place is, as you say, convenient.
- 50 (?ish, maybe not that many) new fighter subclasses
Not interested. I'm actually not a big fan of subclasses. Rather than "customization" my feel has been they make your character cookie-cutter... your subclass character is exactly the same as every other subclass character. And you probably have abilities in your "ability set" that you don't even want.
I understand that 3E tended to have certain sets of Feats that synergized well such that if you were going to make a two-hand fighter or a ranged fighter or a two-weapon fighter you generally took the same Feat Tree as everyone else going that build, but it just feels bad to me to make a selection at 2nd level that locks into the exact same set of abilities with no option to swap them out that everyone else has.
So, yeah, hard pass on this - I have zero interest in subclasses. Giving me 50 subclasses means that 98% of your material will not be applicable to my fighter as it's not the specific subclass I took. Or, if I took a core subclass, 100% of this material is not applicable.
- New equipment, weapons, armor
As long as they are mechanically unique, this interests me.
Tell you what, give me 50-ish new Feats - something EVERY subclass can use - instead of 50 new subclasses, and I'll be happy. A Feat is potentially applicable to EVERY subclass. This is the opposite of subclasses - universal instead of super-niche.
Depends on what these rules are. For example, I'd love to see more advanced combat maneuvers, perhaps tied to weapon classes like axes, spears, etc.
I ought to note I especially liked Second World Simulations' "Master of Arms" book and their attempt to turn 3E's iterative attacks into unique combination attacks - their implementation wasn't perfect, but it was inventive. And a prestige class built around fighting with dual-wielded immovable rods was incredibly fun to think about.
And maybe more than "new rules" a section on "combat tactics using existing rules" in general would be great.
I agree with previous posters; rules for "unique weapon materials" are probably better suited for another supplement.
- Role playing advice, seeds, hooks, etc. to make your fighter your own.
Having 40+ years of experience, I don't know that I'll see a lot new here, but I'm not actively against it. There are far more players newer to the hobby for which this will be new ground and I want to see new players get excited, too.