I think the "problem" with Gish characters is that everyone expects different things from them, so making a definitive class that fits all of that definitions and expectations is near impossible. I particularly like different gishes being subclasses, that explore how different classes interwine magic and pointy metal thingys
That's my own take on this thread: I don't think you
can make a single martial/arcane hybrid class that does it all - it would need to be extremely broad, encompass all available warrior styles and all non-divine magical styles, and have at least one way to blend them... there wouldn't be a core to the class beyond hit dice. You could cover a lot with a good swordmage, but they're already thin on story (and the traditional archetypes were all race-focused), and that would leave a lot of options off the table.
The better answer is an option for combining spell slots with martial attacks that anyone can pick up pretty quickly.
Consider the Sorcadin: it works pretty well as a gish concept because Divine Smite lets you spend spell slots to add to melee damage, does so efficiently, and scales pretty well. At pal2/sor1 you
feel gish-y, and you can then progress in either class as much as you like depending on how you want to play the character. Plus sorcerers have the least rp baggage. But Divine Smite works well with bards and warlocks too, so you don't see a gap in Charisma half-casters. That's already out there.
The int classes don't have that. If Eldritch Knights got a good version of Spellstrike (the Pathfinder Magus feature) this thread wouldn't be necessary. Maybe make a feat? They'd blend well with wizards or artificers, and since there's a lot of types of wizards you'd have a lot of potential options without a lot of work.
Which isn't to say there isn't room for more arcane half-casters (I really like the Janissary idea above) but they can't, in a single class, give everyone (or nearly everyone) what they want, and probably shouldn't try.
(Wis half-casters feel redundant because clerics and druids can cover the space well enough as-is.)