Will there be an option to set up a PvP arena with Crucible?

Or something like DOTA where one player can send waves of
creatures he did unlock in the singleplayer campaign against
the other player in maps generated via Crucible?

If not, is there an API comming with Crucible where PC gamers / devs can start modding
without the need to hack anything? Steam is unforgiving when it comes to changing game
data where it isn't wanted by the developers.