Halogenated PCM analogues have proven fruitful, the only halogenated PCP analogue I've tried so far was just ok and 3-Cl-PCP may be similar. Halogenated PCE analogues haven't been tried yet but I think it's a promising avenue for development. I would expect a much lower potency though.
And then we have the PCPr and now the PCiPr backbone to build analogues from! The reflex seems to always try a 3-MeO + 2'-Oxo substitution because MXE but I think 3-MeO alone, 3-HO, and 2'-Oxo substitutions should be attempted for all the arylcyclohexylamine bases. I think it would be a good way to find fruitful compounds! Halogenated analogues and halogenated + 2'-Oxo analogues should be tried too (though PCP and perhaps even PCPr and PCiPr may have too much steric hindrance for a 2'-Oxo).
Compounds I would love to see
2'-Oxo-PCP (?)
2-Cl-2'-Oxo-PCP (???)
3-MeO-2'-Oxo-PCP (????)
I don't know how possible the synthesis of any of these would be or what their stability would be like but its fun to imagine.....