The finale for season 2 would've been a two-parter, the longest ZIM episode yet. Although the plot had not been decided on yet by the ZIM crew, the idea that Jhonen Vasquez had been toying with involved a rescue mission on planet Meekrob:

Invader Tenn would've been captured by the Meekrob aliens, so all the other Invaders are called in to perform her rescue. ZIM learns of the mission from Skoodge, since Skoodge lives in ZIM's basement in late season 2. Dib also goes to Meekrob, presumably in Tak's ship. While there, a conflict breaks out when it is revealed that the Resisty is in league with the Meekrob.

Apparently, Zim would've been launched into a meteor and Dib would've been hailed as the ambassador to Meekrob at the end.

For my own take on this episode, see the Fan Fiction section and read The Battle for Meekrob.