Customizing Specific Mage Guilds and On-map Shrines

A specific settlement's Mage Guild Levels (or a specific on-map Shrines L1/L2/L3 spell) could have must-have spells (which can be from any alignment of spells at that level), might-have spells (from those allowed for that alignment, for shrines this includes all spells at that level except for ones banned Globally, unless override is checked) and never have spells (which can never appear even if it would normally be allowed).

I totally support this, a pre-defined list of spells would come in handy as well as an ability to make it random. The more things we can define in our maps, the better, including editing the Magic Shrines and all that.
Banning spells can be already made with scripting IIRC.

I don't understand the request. Something along these lines is actually already in the game. For example, Necromancer castles are guaranteed to have at least one of Death Ripple and Death Wave.

To either have non-standard spells forced to appear or to prevent normal spells from appearing. And for shrines, to customize a certain one so that it has a random list to choose from and prevent certain spells of that level from appearing.

I think it's already possible with scripting. There was an introductory video on Ironfist YouTube channel showing the unusual Mage Guild for a Wizard's town.

It probably is possible with scripting, but you'd basically have to entirely reset a town's spells to do so.