I'd like to see Eldritch Council get an update. Play off the idea of an order of Loremasters and Wizards in a tower collecting information. Maybe like an Illuminati of the Warhammer world causing wars so they can snatch rare books. Add in plastic sword Masters and Spireguard plus incorperating swifthawk agents. GW can even use names of important locations from WFB like "Citadel of Dusk, Fortress of Dawn". Their army should be small and compact, but with a heavy focus on the wizard lord and his acolytes.
Nurgle: Rotbloods (think of Khorne Blood Reavers), Pestigors, plastic Nurgle Sorcerer.
Freeguild: Give them back all their artillery, and make them an offensive musket army (kind of short range, bayonets on muskets).