You can be implacable, disgusted, inert, nonchalant and gormless, so why can’t you be placable, gusted, ert, chalant and gorm?
John Rentoul at The Independent has a list of the Top 40 Lost Positives, and someone’s even written a short story using them. Personal favourite: ebriated. I’m feeling totally ebriated at the moment. Must time for a glass of wine …