One approach I have seen used is to have a roadmap which has epics/features shown as circle or ovals whose size gives a rough indication of the work involved. There is either no time units indicated or at the most quarter years are shown.
The idea is to make it deliberately vague, but still give an indication of the possible order in which work will be done. The more precise the roadmap looks, the more people will interpret it as having fixed dates.
Even using this kind of visual roadmap is dangerous as it creates some expectation for delivery. I would deliver the roadmap with a clear message that as you are working in an agile fashion it is possible or even likely that the roadmap will change over time.