Even if you are rolling for spells in the BRB lores, this will be a lore attribute made of win.
Let's assume the Slann takes High Magic and is a Loremaster. High magic has 8 spells, not all of which will be useful in every battle. But each of those spells could be swapped out for any of the signature spells in the BRB. This sort of echoes the High Elf Loremaster of Hoeth, who starts with all 8 of the BRB signature spells. But in the case of the Slann with this new attribute, you will be picking which of the 8 signature spells in priority order, and also have the chance of rolling up something better/more suitable than the signature.
Now at first glance, you might be thinking, "but that means I have to first CAST a spell that is useless in order to get rid of it!" Not necessarily. You will just need to change your thinking to include the REST of your magic phases. Maybe you pop off with fiery convocation in your first magic phase to hit that deathstar unit. After this magic phase, you might then consider, "well, I probably won't cast that again, now I need to shift to combat buffing and debuffing." You can then take something like Wyssan's Wildform or Miasma in place of Fiery Convocation. Or perhaps you are in a tournament and have found yourself drawn up against heavy armor? Grab yourself some metal death dealing or get all character snipey with Death lore.
This would make a Slann with High Magic the most versatile caster in the game, able to cast any of 16 unique spells (reliably) during a single game. Of course, the real number is 64 possible spells, but you can't rely on rolling the spells you want when it is time to pick a new one. I REALLY hope we keep Loremaster, that would fit very well with this new attribute.
All that said, I'd like our very own lore, but I'd be happy with High Magic with the ability to customize our spell deck over the course of the game. Talk about a great caster to take to tournaments!