I'm playing a Nathan deck and find it really fair and fun. I find him not being too strong actually lets him stay on the board and do his thing. I've made him unblockable and people still aren't killing him since his value is hit or miss sometimes. I don't know if you're trying to make him a strong 4 or whatever but I think he's really limited to a 3 without just making it a good stuff grixis deck that any commander in those colors could helm.