Of course it is really nice to play someone of your rank because that way you are almost guaranteed to have a good game. But I miss something that used to be part of most online games before ELO and other ranking systems became popular: randomly playing other "anonymous" players (in the sense you don't know how good they are).
What I'd like to suggest is a mode where:
1. You are matched against players of any level between -10k to +10k of yours (or something like that, I didn't waste more than a few seconds thinking about this so there should be better ideas here);
2. You can't see who they are (so you really can't find out their rank) and you aren't told who they are even after the match;
3. The matches are not ranked;
4. Chat may be enabled or disabled (as an option);
5. Chat is enabled after the game regardless of the chosen option, so if both of you are interested you can then meet each other;
6. These are private games in your profile, no one else can see them!
Although it may sound crazy, I do think this is a very nice feature in some cases.
1. You can play random games for fun, without the fear of being "tested" (yes, that's a thing).
2. You can play against weaker players and end up teaching them something; you can play against stronger players and learn from them.