TwoSix
Dirty, realism-hating munchkin powergamer
Not a perfect fit, but the Magus class has a fair amount of those features.I am curious about both, but I wanted to start by talking about how a true swordmage could be built in 5e as it is, and from there see what is still missing. I think a new class with a purpose built spell list, and some new features to make it really sing, would be a fairly small amount of work.
I'd make it an Int caster who chooses between Strength and Dex just like a fighter, gets cantrips, doesn't get Extra Attack and instead gets something like a bonus action attack when casting a cantrip, and get's either a d8 or d10 hit die, with at least one subclass or Aegis option that boosts durability into real tank range, but other options that focus more on mobility and hitting hard, or control, etc.
I'd probably give the base class a Mark ability, but have the Aegis options determine what it does beyond what a Mark does in the DMG optional rule.
Hit Die: d8
Spellcasting Stat: Varies by subclass (chosen at 1st level). 3 Int subclasses, 2 Cha, 1 Wis.
Cantrips: Yes. Gains 3-4 weapon attack cantrips (like Booming Blade) per subclass. Also can make a BA attack after casting a cantrip at 7th level.
Str or Dex: Can use either, but favors Dex since it doesn't have heavy armor proficiency.
Aegis: The base class can place an Aegis on the target that gives a small, scaling AC bonus. Each subclass gains the ability to remove the Aegis as a reaction on triggering damage to activate a larger effect. This effective is offensive or defensive based on the subclass.