I hope Yorick gets the Poppy treatment, they did her right. I do hope they share their thoughts because some reworks in the past have been terrible. I think almost all Yorick players can agree on a few things though, the mana gating makes for stale build diversity for the most part (everyone but me builds manamune it seems), his lack of mobility makes him more vunerable than it should, and that he could be a bit stronger in teamfighting situations.
The solutions seem pretty obvious really, adjust the mana costs to not drain him in a single volley of skills or make the skills individually strong enough that he only needs cast them one at a time. Make up for his lack of mobility with some kind of massive damage reduction or CC at close range, thus he doesn't need to be as mobile if he can ignore damage or stop enemies from chasing as he walks away. Lastly give him some utlity outside of a faux revive for a teamfight.
Rather than just point out the weaknesses in his current design though, I thought perhaps providing some ideas might help more. I suggest making his sustain close range and frontal cone AoE (still summons healing ghoul) instead of a targetted and semi long ranged, making his Q a skillshot fast moving ghoul hit that still procs an auto reset and extra damage on next attack, and giving all of his skills a free second cast that each have effects that correlate with the base skills. Healing ghoul can be recast after a second to do a frontal cone AoE that heals Yorick, Speed ghoul can be recast a sec later to massively increase Yoricks movespeed toward it (or pull Yorick to it), Slowing ghoul could be recast to pull nearby enemies toward it (like Diana's pull but centered on the ghoul). I feel his Ult should gain the damage reduction and boost from his current passive and a new passive involving being around dying minions or monsters to replace the current one. I would like to see something interactive, perhaps a way to have his passive be able to send the souls of the dead to be his eyes, scouting out nearby bushes and such.
So Riot, help us out here, share some ideas maybe?