I like the tattoo idea, especially for non-wizard casters such as Druids, Rangers, Clerics. I know that kind of misses the point of this thread being about Wizards.
Although for wizards, it would necessitate a third party. To ink the wizard. Other classes could have the tattoos appear or reform through divine/diabolical/nature magic directly onto the skin. But for wizards, of course they can tattoo themselves to an extent but at some point they're going to run out of space and need someone to do their backs.
Unless... When they graduate from wizard school as level 3 wizards having chosen their school, they are tattooed with every spell known to that school all over their body, in a distinctive style that immediately identified the faster as an abjurer, evoker, whatnot.
But as time goes on they gain the ability to access and tap in to tattoos they aren't able to at level one, effectively "unlocking" new spells.
Spell scrolls would be tattoos copied on to parchment that only one who bears that tattoo can cast. A Magic Initiate might have a few tattoos as opposed to a whole body "sleeve" like a full wizard. Or for spells of level 5 and higher, a spell scroll would need to be a piece of a wizard's skin...
Raises couple of questions:
Big trade in selling dead wizards' bodies...what about safe disposal?
Also, how would other people view tattoos? Avoid them (maybe forced to by the magicians guild)? Or fake them?
Alternatively as a wizard reaches