I would love to see implementation of a "Spectator Mode" and supporting queue(s) and such.
If you want this to become an "eSport" contender, please enable a robust spectator mode, and supporting social frameworks:
1) Games can be "private" or "public."
2) Public games can be "spectated" [watched] by others.
3) There should be some method of finding games to "spectate" [a queue].
4) IMO, "Spectator" queues could list vital statistics for the match(es): (Usernames involved, user ranks [if applicable in ranked play], match duration so far [1:10:15, etc.], current life totals on each side, current # permanents in play on each side [maybe broken down by type? (artifacts, lands, creatures)], current number of spectators, etc.)
5) In a match, while spectating, it would be cool to have social features like "follow player" (IE, if you like their decks & play style, maybe you could "follow" them to spectate future matches), "Add Friend" [don't interrupt players, but add to notification/request queue, maybe with a note "friend request from a spectator of one of your recent matches"], and maybe chat with other spectators [maybe not with the players, so as not to distract them?].
6) It would be cool to be able to see "followed" users' current matches somehow, so you could spectate future matches. Kind of like friends list, but just for folks you're "following" as opposed to friends with? Or maybe a social list of active "spectate-able" matches divided up into "Friends"/"Following" [separate from regular public spectation queues, or a sub-division just for people you're following or friends with as opposed to random matches in a general queue].
I feel like this would be a cool way to see others' play styles, find potential "influencers," get deck ideas, and be more social in the game.

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MGmirkin#16543 commented
7) to whatever degree possible, "spectating" should not impact game performance for either of the players being spectated. Like, it would be more like a "broadcast" to other players that wouldn't hopefully impact like play or performance of say trigger resolution or other things. Since Spectators wouldn't be part of the game itself, ideally they shouldn't have any impact on it, by and large, if at all possible. More just passively viewing and hopefully not bogging down or lagging the servers, etc., in a way that would negatively impact actual play...