Won't argue with that... for now
There exists a game, called "Neverwinter Nights", made by wizards of the coast, the second greatest after the blizzards imo and founders of DnD, where "known spells" issue for wizards resolved as "level requirement": after reaching spellcasting level 3 a wizard can learn all 3-level spells from scrolls and gains some of them on level-up.
Within RifE it can be quite easily done by acquiring on lvl-up a special promotion called "effective caster level" or whatever, that will enable caster to learn this level spells from others, libraries, lairs and stuff, but only be able to cast these when required quantities of certain colour manas are available, or even more than one caster simultaneously(coven-castable spells) more than one caster simultaneously.
For example,
"Worldbreak", requires 3 fire, 3 death, 3 dimensional, 3 chaos mana and 13 lvl 3 spellcasters (archmages) to wreak the world to hells and kill em all.
That would remove EXTREMELY annoying me possibility to spam ultra-powerful global spells like stasis or arcane lacuna on power basis of tiny village amidst untamed wilds at gamestart.
(Yes, leader is powerful spellcaster of himself, yes, he generates some mana, but to launch something truly powerful like stasis, he will need support spellcasters and lot of worshippers/coloured mana nodes)
And based on percentage of known spells, their caster shall autoreceive bonus promos, like "Adept of life magic" or "Omniscient Master of Arcane", which i see is already partly implemented.
Resume: New spells should be not gained one at lvlup, but learned from special sources; at certain levels there should be gained an ability to learn certain spells, and possibility to cast known spells should be mana/number of casters with required level based.