What else would be found from the raided mage's guild secret workshop aside from the medalion?
Probably new spells.

Adding the traditional elder scrolls spells through spellcrafting like the antiquities system is the way to go: spells from the schools of Alteration, Destruction, Conjuration, Illusion, Restoration and do not forget Mysticism.

No need to have any balance breaking spellcrafting like the single player games had, just allow us to have our fireballs and calm spells. Adding a search around the world and a mini game to unlock said spells.

I really don't think they're gonna add spellcrafting this chapter based on one line from that letter.

Full custom ability customization sounds like it'd be a nightmare to balance, and you just know players are gonna break it or come up with nonsense like 'BiS spells'.

What I would totally be down with (and is imo a much more feasible implementation design / balance wise) is something like a spellcrafting lite system that allows only minor thematic or visual changes mainly meant to customize your character even more. Like changing the colour / appearance of a spell, or changing elemental damage, like fire to frost or lightning, maybe even to synergize better with armour sets you wear. Or perhaps change the cliffracer warden spell to look like a spectral bird instead, or change werewolf appearance to wereboar. Or change that damn overused jabs to look like a series of rapid strikes with your weapon instead (please do that one day ).