After reading the whole thread, some points:
INT Martial:
WoTC doesn't care about the magic / martial divide very much... Arguably the monk is a WIS based pure martial, but they don't have a pure martial based on CHA or a pure martial based on INT. . But players do care! There is a desire for a non-magical character that is dangerous for a reason other than brawn. Yet, how and why are they dangerous exactly varies by class. Well, if they are really precise, quick, or really stealthy, that is Rogue (DEX). If they are really disciplined or mobile they are a monk (WIS, DEX). If they are really sensitive and atuned to the advantages of the terrain they are a ranger (WIS + woops, WoTC gave them magic after all, sorry). If they are dangerous because they are building machines or poisons to win they are an artificer (INT, Half caster snuck in again because of course tech is magic). If they are dangerous because they are organizing their side to fight better together they are a ......... WARLORD / Tactician (INT or CHA (maybe you pick?). If you had a spell-less BARD, they might be your CHA warlord.
So I think Warlord is the biggest missing class. I would assume INT based, but you could also have and INT and CHA MAD if you want kind of a spell-less bard take on the Warlord. Just don't give it spells. It should have maneuvers like the Battlemaster, but focused on givining the party an edge. And it was retroactively the class that Elgin had in the D&D Movie.
Monster Class:
I think the argument for a non-caster "Monstery" Shapeshifter / Beast class is pretty interesting, since it is such an archetype with so many sources. There is a design space here the size of the monster manual, but balancing it will be rough. There is also some argument here that it would be best water down the idea into a barbarian subclass.
GISH:
I would argue that the gish has been done enough ways people should probably be happy with it. The artificer, because of its (unusual for D&D) tech flavor and absence from the PHB, doesn't fill the INT halfcaster space as well as the Ranger does the WIS half caster or paladin as the CHA half caster do. I would have been happier with a non-tech half caster, with special magical maneuvers, but I don't know if we really need it.
The Wielder class people have been suggesting is just a Warlock Pact I think.
The Witch: in my mind is a great aesthetic archetype but a fuzzy mechanical one. What makes a witch hard in D&D is not that there is some clear "witch power" that we don't yet have mechanics for, but rather that witches are mostly defined by their gender and status as mystery outsiders and historical witches aren't cleanly on either side of the D&D hard barrier between divine and arcane magic. A witch might have spells smattered all over the Druid, Cleric, Wizard, Bard and Warlock lists. Then this unparalleled spell diversity would have to be balanced by giving them some power limits of another nature -- ones that hopefully would not ruin the enjoyment of playing the new class.