I'm not really convinced a unique Slann lore is either totally necessary or particularly fluffy. From a mechanical perspective, rule book lores have been heavily toned down compared to the BRB. You can take that as you will: its possible that its a deliberate design decision by GW that unique lores are weaker so that there are fewer hangovers between edition changes (See: Lore of Tzeentch & Lore of Dark Magic, both of which are highly effective with extremely low casting values now that you take magic level in to account), and the BRB rules are the default go-to ones, making them easier to balance between editions.
Either way, I don't see how a unique lore could be beneficial for gameplay terms - what could it do that Life or Light don't already do, and do very well indeed?
Additionally, I have always felt that unique lores are a reflection of the race that uses them, developed in particular circumstances. I don't see Slann or Lizardmen as particularly intuitive or creative: thats why the Old Ones made Elves and Men. Lizardmen are the specialised redundancy. They are very, very good at what they are told to do - ie, the standard 6 lores of magic - but not particularly good at coming up with new things to do.
I look at Skink lords the same way. Other than a level 3 who has access to other lores than Heavens but cheaper than a Slann, what other purpose could a Skink Lord serve? If you want a kitted out combat Lord, Oldbloods are going to be better than a Skink. If you want cheaper combat character, Scar-Veterans fill that niche. If you want to dominate magic, Slann are going to be better than a level 3. Even if you just want to be competent in magic, two level 2s or a 2 and a 1 and a sprinkling of arcane items does the same job that a level 3 does. Look at the number of lists that use 3s these days and you see hardly any, because its a very limited niche that hero mages fill. Sure, its an additional option but you have options in hero selection.
What I would like to see are new options of a completely different kind, with heroes filling brand new roles rather than existing ones that we have. Lizardmen don't have the equivalent Warpriests, Witchhunters, Engineers, Necrotects, Banshees and so on. Lets get some of these character options that add synergy: Relic Priests that grant bonuses to nearby units, Skink Prophets like Tehenhauin with unique bound spells, and so on.