ah, the thing I can't do.
found my notes they are likely only a mediocre idea so do not put too much in it.
first, why would an arcane half caster appear in-universe?
fairly simply the most likely set of people to generate the need for this kind of class is a magocracy as they would need some less squishy enforcers of their will that can not just be dispelled by other magic.
eventually, they would want something with more power than a fighter but less of the restrictions of the paladin. thus teaching fighter magic would giver rise to things like the eldritch knight, arcane trickster and the echo knight. as anyone could tell they would not quite be what they(and we)want.
this is the bit that needs story may be made with something possibly enhanced somehow?
creating something as useful as smite but very different
thinking about things which damage a bit but are mostly ways to inflict status effects and debuffs. no idea what to call it but its basic use is structurally similar to smite in basic mechanics but used at ranged?
basic level up ideas
Here is where things start getting tricky as level up abilities are a heavily mechanic based thing.
okay here is the idea people love the customisability of the warlock so why not have a somewhat lesser variant of that as each subclass level up, given the thematics I set up for this class it would also work nicely.
concepts for subclasses
each subclass spell level only goes up to five like other half casters but each one is based on two types of arcane magic.
I must not base the subclasses on just the logical combinations of the magic schools. I suck at names but here it goes.
1 evocation and conjuration lots of bouncing around the battlefield like the swordmage but with a lot more elemental damage attacks and summons backing them up when numbers are needed. most likely designed to be medium armoured the standard one first built designed to conquer territory and remove external threats.
2 abjuration and necromancy your arcane juggernaut, hard to hurt, heavily armoured, can regain health by hurting its enemies, lots of necrotic damage, and ways to hurt your enemies soul. basic in-universe function somewhere between mage policers, heavy troops and undead killer.
3 illusion and enchantment closer to rogues than fighters, stealthy with seemingly endless ways to deceived others and exploit the mind. need a magic item, spellbook or scroll "collected" get these guys on it, need intelligence on a place get one of these to handle it, need a population deceived into rebellion this subclass was built for it. basic inspiration what would a secret agent be if given magic and they were made for a fantasy setting. I think my concept here is weaker than I would like but lots of people would still pick it.
4 divination and transmutation. medium armoured will likely be getting most of its damage form weapons but is the subclass for people who find reality unsatisfactory(possible class motto reality is an illusion and I can prove it.) with the highest number of ways to flat out modify the situation for people who live to make combat "interesting" some subtle luck manipulation and giving them some chronomancy powers late game? think they arose as an attempt to make a better battlefield fighter then they mutated into a cult believing that reality is not real(whether or not they are right is dm discretion