If it would like to be CPU only for a while then theres
M7M (v2) PoW hash function from Magicoin which is still CPU only I believe
I was following Magi religiously when they first started, but after a short amount of time this same situation occurred, now there are GPU's raping the mining pools :/ I'm not sure if they took any steps to circumvent that, but GPUs kicked a huge amount of people off the pools

how about argon2 coin (AR2)? ... I've read it is quite hard to implement on GPU's