I feel like "canon pair" and "rare pair" are two different pairs of shoes. what is in canon text must not necessarily be popular in fandom.
how I judge what is a rare pair in any given fandom (just my way to do it, not a universal definition): I look at the ao3 page of the fandom. Example: Captain America (movies). Then I open the Relationships tab in the side bar. This lists the ten most tagged pairings in the category. Imho none of these can be considered rare since they are the top ten, even if #1 probably got a lot more works than #10. For Cap movies we got the following top ten pairings:
James "Bucky" Barnes/Steve Rogers (29114) James "Bucky" Barnes & Steve Rogers (5203) Steve Rogers/Tony Stark (4647) Peggy Carter/Steve Rogers (2767) James "Bucky" Barnes/Reader (2258) Pepper Potts/Tony Stark (2220) James "Bucky" Barnes/Tony Stark (1988) Clint Barton/Natasha Romanov (1957) Steve Rogers/Sam Wilson (1880) Steve Rogers/Natasha Romanov (1680)
Steve/Bucky has more fics than Steve/Sam but the latter is still in the top ten and therefore not rare to me. Same goes for Clintasha, which is probably a side pairing here (same with Tony/Pepper) but used to be huge huge huge in Avengers fandom aomong people who prefer het.
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how I judge what is a rare pair in any given fandom (just my way to do it, not a universal definition): I look at the ao3 page of the fandom. Example: Captain America (movies). Then I open the Relationships tab in the side bar. This lists the ten most tagged pairings in the category. Imho none of these can be considered rare since they are the top ten, even if #1 probably got a lot more works than #10.
For Cap movies we got the following top ten pairings:
James "Bucky" Barnes/Steve Rogers (29114)
James "Bucky" Barnes & Steve Rogers (5203)
Steve Rogers/Tony Stark (4647)
Peggy Carter/Steve Rogers (2767)
James "Bucky" Barnes/Reader (2258)
Pepper Potts/Tony Stark (2220)
James "Bucky" Barnes/Tony Stark (1988)
Clint Barton/Natasha Romanov (1957)
Steve Rogers/Sam Wilson (1880)
Steve Rogers/Natasha Romanov (1680)
Steve/Bucky has more fics than Steve/Sam but the latter is still in the top ten and therefore not rare to me. Same goes for Clintasha, which is probably a side pairing here (same with Tony/Pepper) but used to be huge huge huge in Avengers fandom aomong people who prefer het.