Rangers have the problem that a lot of their abilities are situational and many of their spells aren't a ton better than just using their weapon.
If you consider all 3 phases of the game, IME a Fey Wanderer Ranger with the shadow touched feat and a 20 Wisdom is the strongest class there is from levels 9-14 or so. They have no weaknesses or liabilities they are amazing at social skills, beating even Bards if they put their expertise there, they are the best wilderness/exploration guys in the game, they are good damage dealers in combat using weapons and their concentration-free Summon Fey and Cause Fear with beguiling twist are awesome controllers and breaks action economy. As icing on the cake, add in a few hundred goodberries after a day of no encounters and you have an extremely strong character.
Awesome controller, Awesome Face, Awesome explorer, Great Hit points, Good damage dealer, Servicible healer
In terms of being a Gish, they are more caster than fighter most of the time, but they are really powerful in those levels (and no slouch at earlier or higher levels either).
You could probably just do an arcane paladin sub-class and make a very good sword mage. Though I think Eldritch Knights already work pretty well if you prefer the emphasis more on the sword than the mage.
EKs, twoArtificers and Bladesingers are all Arcane Gishes, which makes the landscape pretty crowded already. That said if you want a half caster I agree the Paladin Chassis makes the most sense, and I think I posted an example of this earlier (if not here than on another thread).
The problem I see with a lot of swordmage proposals are the mechanics are too powerful or at least on the more powerful side. The claim is not that the idea of an Arcane Gish is OP, but rather that the implementations offered are.
People say they want a Swordmage and want the flavor, but then they want that character to be on the high end of the power scale, up there with a Bladesinger instead of on the lower end and more equivalent to the Monk. This creates a problem because this is not playtested. I think if you are designing a new class you should shoot for the lower end in terms of capability and power, but most of the suggestions I have seen are on the high end powerwise.
In any case, here is what I posted earlier using the Paladin Chassis through level 10:
Intelligence and Dexterity saves, select spells as spells known for a half caster but allow selections from either Paladin or Wizard lists.
Level 1: Trade Divine Sense for proficiency in Arcana or Acrobatics. Trade Lay on Hands for Ritual Casting
Level 2: Fighting Style (allow all fighter styles), Keep Divine Smite (maybe call it arcane smite)
Level 3: Trade Divine Health for two Wizard cantrips.
Level 4: ASI
Level 5: Extra Attack
Level 6: Trade Aura of Protection for ability to use your weapon as a focus for spells
Level 7: Get a third Wizard Cantrip
Level 8: ASI
Level 9:
Level 10: Trade Aura of Courage for the Bladesinger version of Extra Attack
If you did that and made it flat with no subclass, I think that combination would be where it needs to be while incorporating most of the flavor people want through spell selection . By making it a spells known caster, removing Aura of Protection and subclass abilities you keep it from eclipsing the Paladin in power even with the larger spell pool. By pushing the Bladesinger extra attack to level 10 you keep it behind the bladesinger at least until that point.
