We’ve had MTG threads in alt gaming in the past. Several, in fact.
Having it’s own section won’t increase the number of posts on the subject. Either they want to come here and discuss it or they don’t. What section the discussion takes place in has no bearing on the willingness of them to register here. If you say “hey we’ve got a section just for MTG” it won’t be any more or less attractive than saying “hey we discuss this all the time on our forums”.