You've mentioned a dislike for the rule of one and the 5 turn systems on a few threads before. How would like the game to be played vs how the game is currently played?
Obviously everything with involve a general rebalance and pruning as well seeing as it's bound to have some consequences.
Rule of one:
To counter mortal wounds spam I'd introduce the concept of physical vs magical damage and give every unit a physical and magical save. As everyone now has a save spell offensive spam shouldn't be much of an issue. Also it has the benefit of (partially) removing the need for mortal wounds (or excessive amounts of rend) as it'd be possible to deal with a unit that has re-rollable 2+ physical save by simply throwing magic damage at it. and take advantage of it's worse magical save. And it allows for interesting units by specialising them in one type of damage (both offensivly and defensivly) and would even allow for spells doing physical damage (say you launch a rock at someones face) to allow for some interesting strategies. This concept should also be applied more broadly to damage in general.
To counter spell stacking: the same buff doesn't stack on the same target (and imho, this should also go for certain command abilities, though some command abilities could potentially get the option of "pay more points for a more powerfull effect" seeing as some of them do create interesting uses of certain units)
To counter nonsense like Nagash: don't make nonsense like Nagash who on his own can cast more spells than the entire enemy army combined unless he faces a tzeentch army (and even then it's gonne be a close call unless you play very large games). Seriously who though it'd be a good idea to have a model like that?
As for the rule of one surrounding artifacts, I've never really seen an example of why picking the same artifact twice would actually be problematic (and I can't really think of any unless the artifact is just broken to begin with at which point maybe fix the artifact). So that one I'd just remove. Possibly with some exceptions for particularly powerfull artifacts.
In my opinion that should solve most of the potential issues with spells in their current form.
Also, I'd try to create more (spell) effects like some of the new slaanesh abilities that introduced an element of choice for the target (e.g. the seeker's dark temptation ability). Those could be extremely fun and the choice could be used to keep the ability in check, especially if the choice is utility focussed or gives opponent a potential advantage.
As for the 5 turn system. I don't think that can really be done without making it a completly different game. And the few ways I can think of that would at least sorta work would also make the game take way too much time.
Edit: I forgot to mention; a mechanic like kroak's celestial deliverance progressivly becoming harder to cast with each attempt is something I like as well