I did something like this a while back using Paladin as a base class just changing smite from radiant damage to elemental, revamping the spell list, and swapping out abilities like Lay on Hands that don't fit. The paladin smite plus the various "smite" spells sounds exactly like what you are looking for in terms of magic that adds to melee attacks instead of replacing them. It is already a fighter-ish half caster, so just add unarmored defense (maybe swap it for Lay on Hands) and it is pretty close to what you are describing.
I'd also say without heavy armor there is no reason to use a Str build instead of a Dex one.
I think I’m not gonna bend on ritual casting. Might give them wizard ritual casting via their spellbook, but a normal spell list/prepared spells system.The UA Stone sorceror might be a good place to start. - Non-armour based defence, lots of Smite spells etc.
This class sounds like a half-caster, or perhaps use the 4 elements Monk spell system. Comparing with a Kensei monk for balance and mechanics would probably work.
- Deciding on resource model is probably the first decision that you should make.
Half-caster using spell slots like Paladin or BS Artificer? Monk/Warlock/BM Fighter analog with short-rest refresh? Mostly at-will abilities?
Is this a class that calls fire into their blade and then fights with martial skill? Or someone who doesn't fight that well, but every so often can blast a lightning bolt out of their sword or smash the earth to knock people over with the shockwave?
Extra Attack - bear in mind that you can either have an extra attack at 5th level, or have an ability that boosts your attacks kick in at that level and have Extra Attack at perhaps 11th. Stealing the Eldritch Knight''s ability to combine attack with cantrip could work.
Spellbook might work with the base class, but I think that Ritual casting would fit the Scholar subclass rather than the base class.
Access to Mage armour, Absorb Elements, and Shield spell?
Or maybe a "defensive smite"-like ability where you can burn spell slots to power a parry that reduces damage like the battlemaster maneuver. D10+Int for a 1st level slot and increases by d10 for each slot higher than 1st?
That’s why I proposed an option that makes your AC 10+int+proficiency. It’d start at about 16, and cap out at 21. If that’s too good, for some reason, we can drop it down to 8+, which goes from about 13 to 19, and would want for a second feature or a shield to have good AC at low to mid levels.Paladins are already perhaps the strongest and most versatile front line fighter type - especially so without feats. I'm not sure we want a new paladin strong class introduced?
I agree that heavy armor or perhaps medium is really what opens up a strength build option. Without at least medium or a substitute that's not reliant on dex then strength is the inferior build every time.