Marathon Mode would be a little extra thing for people who want to play maps non-stop. You'd be able to set criteria like Star Rating, Length, and BPM and you would be able to set mods to be put on all of the songs you play. You might say, "We already have Marathon maps. Why do we need this?" Well, say you have multiple Touhou songs that aren't in a marathon. You could use Marathon Mode to play them as if you're playing a Marathon map. And plus, who ever plays only one map?