Unless PF2 is a roaring success, I don't see much hope for the Advanced Players Guide (no matter the name) coming from WotC.
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I rather think the opposite: if PF2 horrifies and repels it's existing 3.5-die-hard fanbase, a 5e supplement aimed at enabling more 3.5-like play could well finally bring the back to the fold.
In fact, what 5E has done to revitalize the Fighter class cannot be understated.
Heh, I don't think you meant to say that, but it's so true.
The 3.x fighter was overshadowed in terms of power/versatility by Tier 1 classes & optimal builds in it's native system, but it was an elegant, customizable, dynamic design, arguably one of the best-done class designs in D&D history - it's just hard to appreciate the bonsai when CoDzilla had breathed radioactive fire on it and stomped on its ashes. ;P OK, it also wasn't great at skills, but if you really wanted a background skill you could devote half your 2ranks/level to being whatever sort of crafter appealed to you - or fairly smoothly MC to another class, even an NPC class like aristocrat or expert, to get more skills and paint a picture of a pre-adventuring background (as could everyone, of course).
The 4e fighter was arguably the star of it's Defender role, and the best-supported class (for the only time in the game's history, the most-popular class was getting the most support, weird, huh?) until Essentials started lavishing its attentions on the Wizard, though even Essentials expanded the fighter, nominally - specifically into the Striker role with the Slayer sub-class. Not only that, but the old fighter concept it wasn't really contained in that one class, the Ranger and Warlord were essentially fighters, as well, in that they were covering non-caster archetypes who would have had to have been done (often badly) by fighter builds, in the past. And, of course, it could use backgrounds (and Themes and feats) to fill in the RP/backstory details (as could anyone, really).
For that matter, the 2e Fighter was a DPR king, who was tough as nails, and could even manage excellent saves across the board at high level, staying virtually relevant with the right collection of magic items - and it even had Kits that could backfill a little RP/backstory detail and give the odd perk or two (but, if you were using their Complete book, so could every other class).
What's the 5e fighter really bringing to 'revitalize' that storied history? It's not elegant in design, isn't backed up by other fightery classes, so is back to pulling prettymuch the whole host of non-casting archetypes by itself (really, with just the Champion & Battlemaster in the PH, and the Champ doesn't cover much). It does come through with the 2e Fighter's high DPR and relative toughness, though not it's later-game saves or expected magic item collections. And, it can take backgrounds to fill in RP/backstory and add a skill, but, of course, so can everyone else.
It's hard to understate how much revitalization that represents. "None," for instance, still over-states it. ;P
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Seriously, though - and sorry for spilling so many electrons on a typo - a crunchier rules expansion could add so much to the 5e fighter! Maybe it wouldn't be practical to try to add back everything great the fighter had going for it in 3.x/PF/4e/E, but it can certainly be given a lot more, more detailed, options, and tremendously more customizability, versatility and interest - hopefully w/o falling into the 3.x trap of being a great design at the bottom of a heap of OP designs!
Oh, and thinking of the 3.x fighter design reminds me: 'advanced' class designs should work better with modular MCing. Not as in 'enabling dips,' but as in meshing smoothly, so that a 5/5/5/5 character can have a shot at being as good as a 20. The way 5e handles MC'd casters & cantrips, for instance, works smoothly compared to the way 3.x handled MC'd caster/caster builds (remember the Mystic Theurge?).
Oh, and while the Mystic Theurge was exactly what a PrC shouldn't have been (a MC-enabling kludge), PrCs would be a nice mechanic to add, very evocative of 3.5, adding a lot of options. Though, I do feel they'd've been better as a core rule (with few/no examples in the PH), with actual PrCs appearing exclusively in setting & adventure material, as tie-ins the players could use to make their characters fit the campaign that much better...
