It is an idea I have been coming back to a few times but usually giving up on after a few hours of browsing the possibilities.
The general character idea is a skilled swordsman, who after some events made a promise to never use their blade (or any weapon... or fist) again. They still carry it around but keep it sheathed, using it as a spellcasting focus.
5e, 1-2 uncommon magic items, starting around level 5-6 with no additional wait-time for the build to 'activate', with no worries about it sucking even more when reaching later levels.
Multiclassing is an option but the martial needs to be the one dominating.
Most ideally, I was looking for something that would utilise a Fighter in a non-obvious way, mixing it with a spellcasting class/es for the rest of the levels or grabbing feats / race that would give them enough cantrips and tricks to not hold the team down.
I'm also fond of doing 2 levels in Stars Druid for the bonus action attack but metal armour restriction makes it slightly redundant.
An obvious answer that would make all of it easy would be just Flavour or a single level dip to have armour and weapon proficiency as a spellcaster but I actively want to make it mechanically dumb, while still working.