"There's a big debate if we should use subtypes outside the window of where they are mechanically relevant to allow interactions in eternal formats. That card (Thran Portal) was us dipping our toes in the water and trying it out."Personally, I think this should be done more often! Usually when stuff like that is added, it's expected to have synergy with nearby premier sets (Thran Portal actually made me think gates were going to be mechanically relevant in MOM), but this is only because this was how it's been used in the past.Not only is backwards compatibility one of my favorite things, but having things like this happen more often means the few that are in fact throw forwards for premier sets don't get clocked immediately as hints for what is to come.

The biggest con is it creates expectations that don’t get met.

You've stated that you don't use mechanics or subtypes in a set if they don't matter (like printing a card with a landfall ability but not labeling it as such). However, Thran Portal has the Gate subtype. Does this represent a shift in design philosophy or are Gates becoming Deciduous or something like that?

Magic just breaks its own rules from time to time.