Honestly, throwing a slow into his Kit is rather pointless, considering that Rylai is for most intents and purposes a core item. It synergizes too well with his passive and Liandry. Personally, I'd like to see a change that makes his ult less victimized by RNG/Fizzling out (Zhonya, Playful trickster, windwall, etc). And give his E something more significant outside of setting up his stun or just being follow-up damage from a W+Q. It's rarely used for it's AoE splash damage outside of lane phase.
Another reason that Brand just isn't seen is because he's a high AoE damage/burst mage. That's literally it. If he falls behind in the game, he quickly becomes pretty useless because his kit lacks utility. The comparison I see a lot is Orianna... They both deal excellent damage. Except Orianna's ult has more potential to deal her damage consistently, it has a lot more utility, and a lot more impact on a team fight. Coupling this with her shield/MS/resist buffs, why would you pick Brand over her?