You might be thinking now, " What is a mixed spawn map?" A mixed spawn map is a team map in which some spawns from one team are intermingled with spawns from the other team.
As an example, consider Setons. Traditionally, Setons is played in 4v4 with all 4 top positions against all 4 bottom positions. A way to play this map with mixed spawns could be Team A having top front and air slots and bottom rock and shore spots. Team B would therefore have bottom front and air as well as top rock and shore.
Mixed spawn games tend to be more fast-paced and focus more on early game T1 rush tactics, and they discourage ecoing and t3 arty spam. I think mixed spawns would provide good contrast to the exclusively unmixed spawns used in TMM now.
One note, however, is that in 3v3 mixed spawns, probably mixed spawns are better of the third man on either team (the one spawning closer to the enemy team) shouldn't be totally surrounded by the three players on the other team.