Looking back on it, what changes would this thread make to 3.5 to make it a viable alternative to 5e or OSR?
You know, aside from “buff martials so they can do their jobs” standard.
Honestly? I don't think 3.5e can be saved.
While I'll sing 3.5e's praises up and down for coalescing all rolls into a single [d20 + modifiers vs. DC] roll instead of the weird mish-mash of dice you had to use in previous editions, in hindsight it (alongside the internet in general) fucked up expectations and how people played the game. Because it changed the scope and framing of the game.
I was around on the run up for 3e's release, and looking back at it that edition changed the game from "what you can do with your character" to "what your character can do for you". It allowed people to lean really hard on "build". The concept of character builds (and the associated munchkins that abused them) already existed in AD&D, of course, but with more options available to everybody and more options for communication with the spread of the internet, gradually a lot of D&D discussion became more about what you
could do with a character, what you could stack, and the best way to stack, as opposed to what you had actually
done with the character in-game. That and a lot of things that should be taken for granted (a Ranger or a Druid being able to forage for food, for example), became tied to skills and some classes were completely shafted for those.
Ironically, giving people more options in rules (why simply be a Fighter with a longsword can you can take five feats and become
the Fighter with a longsword?) resulted in people focusing far more on these rules as the central element of their connection to the game. Think of how many people out there talk about builds they've never played and never will because there are only so many weekends in a year and no GM near them wanted to have to deal with someone whose entire existence is poring over splats finding spells and combos to trivialize every encounter. A 3.5e or 5e player's connection to the game is
different (not better or worse, so long as they're having fun) to someone playing AD&D or OSR, because they're doing different things with the system. Which is also why you see so many OSR converts talking about their system of choice clicking so much better than modern D&D: some people want to focus less on build and optimization and skill rolls, and more on how to get their character to interact with the world in a more organic way. Which you can still absolutely do in 3.5e/5e, but it's sort of in spite of the system rather than being supported by it.
So, yeah. I would rip out a lot of shit, a lot of rules, a lot of subsystems (skills just go, replace them with skill proficiencies so your character
knows how to do shit instead of having to roll for everything and the GM defines the exact results, for example), keep the unified roll system, cap HP growth past level 10, and make spells 7th level and above require considerable effort to learn. It's honestly way too much work, so I'd just go play something else instead.