There are only 2 9th level spells I can see being difficult to implement, astral projection and wish. That is it. Even them two could be done with limited functionality.
The rest either deal damage, turn you into something else (like true polymorph and mass polymorph) heal you(mass heal) or give an effect like invulnerability(literally called invulnerability) for 10 mins, give you advantage on every roll while giving your enemies disadvantage(foresight) ect
Some of these spells have already been implemented in previous D&D franchises and pathfinder (which is D&D 3.5 basically). They worked fine there, I dont see why Larian cant do the same.
But by all means, please do list the spells you feel would be hard to implement? I am very curious.
True Resurrection (how would you make it worth casting over regular Resurrection); But honestly, most of the harder spells to implement I think are actually level 6-8 (not level 9).