Ooo, I've been tinkering around with something like this for a while -- interesting to see a different take on it.
Hmm, I'm inclined to agree you've over-powered some of these. Giving the Tzeentch Magister all spells to start with is a massive boost, especially as a campaign develops, & they can just take extra skills instead of using those advances to learn new spells. But giving the Possessed the spells as well means they can start with three spellcasters! I appreciate that fits the theme for Tzeentch, but is likely to unbalance the warband.
Instead, I'd recommend maybe giving the Magister two starting spells, maybe an extra casting skill, & adding an 'apprentice' hero who starts with one spell. Then they only get one Possessed (since they're no longer Cult of the Possessed, that seems fairish), who might be their demon-host.
Nurgle & Khorne seem more solid, & less beardy. For Khorne, I'd also suggest adding Frenzy to all heroes, & maybe devise a 'bloodlust' rule where they might turn on their friends. Oh, I just remembered! Check out the extra rules on the 'Executioner's Hood', Mordheim Rulebook Magical Artefacts table, p141. That'd be flavourful for Khorne, since K-dog cares not from where the blood flows.
It seems like you're struggling to come up with anything really characterful for Slaanesh. I know I struggled with that -- what Slaaneshi stuff can you do on a battlefield? I did have an idea for a 'sadist' special rule/character, who starts weak but gains power from each wound that happens around them, but I've never fully worked it out.
Hope to see more on this in future!