regarding the topic:

At the current rate we have like 3-4 months per Series. In 1-2 years thats is maybe 5-7 Series. Then after that time just call it something like Series 1a and 1b,2a -2b etc., let them have different rewards, let players pick which one they want to do, they can only choose one at that time. So either you play to get e.g. the Archfiend Set again, or you get the new glam set of that season, if you choose the Archfiend Set you will have to wait 1-2 years to play for the other Set again, but then there would be another 3rd reward already for Series 1c.

If you miss out completely some of those 5-7 Series per cycle, you would be allowed to continue with 1-2 missed Series after you completed the current one, to let people have some catchup mechanic when they didnt play for a longer time, but this shouldnt include all Series, there has to be some downside to it if you don't play for a very long time.

Fomo is alright as concept for Ranked imo. But not for this casual Series structure, it doesn't need to be that restrictive.