I'm unable to post drawings, so please forgive me, but I'd like to suggest the following compounds:
Replace the piperidine ring of W-18 with 5-nitrobenzimidazole, with the side chains attached the same as in Etonitazine.
1-phenyl-4-PCC (or I guess 1,4-diPCC). This PCP with a 4-phenyl-4-hydroxy substitution on the cyclohexane ring. 4-PCC or just PCC is the name given to the primary metabolite of PCP, which has that 4-hydroxy. IUPAC, if I remember my conventions correctly would be 1,4-diphenyl-4-(1-piperidinyl)cyclohexanol. I'd also be interested in that 1-phenyl being replaced with a benzyl and phenethyl. I imagine that the latter will be quite potent as its pretty close to BDPC, or Bromadol, but the desire is for potency between 1 and 3x morphine. Something strong enough to make for plenty of doses per gram, but also weak enough that half a milligram isn't the difference between fun and funeral.
Looking at etonitazine more, I wonder if some sort of cannabinoid-opioid combination molecule could be acheived. The chains aren't optimally placed for cannabinoid activity, but I bet something could be worked out.