Yes, addictions medicine is becoming increasingly popular amongst 5 year grads. I know some ED docs who do clinic (mostly rapid access addictions clinic) and also do inpatient addictions consult service, in addition to regular EM practice.
Some people did do this as a fellowship year during residency, however, with the transition to the CBD residency curriculum, most of your "area of interest" time is now during your R5 year (since R4 is now the Royal College year). This makes it a bit more challenging to do a full 1 year fellowship during that time (as you also need to apply for jobs, work on transitioning to practice, etc..). It's still a bit unclear how this will work.
There are still a broad array of interests that people choose. Ultrasound, trauma, sports med (which can also be clinic based), med ed, transport medicine, PEM, geriatrics, resuscitation, etc..