TL:DR

They were going to do a storyline about Sam and Dean realizing that they cannot save everyone, and that they should at least try to warn people, so they go public with proof of monsters. It was originally going to be a season finale with the next season focusing on the two in more mentorship roles rather than main ones. They did not do this because it would have “blown up the show in a very odd way” and would not have been a good idea in the longrun, most likely destroying the show.

They also mentioned that they were originally going to bring back the Antichrist character as a bigbad(?) but did not for undisclosed reasons.

The third thing they mentioned was wanting to do more “international plotlines”. IE Sam and Dean traveling to other countries to help out sometimes and more international threats, but it would have ruined the Americana vibe and theme to the show. They admit the BMOL dropped the ball a bit but they wished to expand upon international hunters more.