So, I'm playing a sorcerer (go, Tutu!) and I'm planning on having Imoen, Coran, Ajantis, and Yeslick. My question is - who do you guys think I should have for my sixth character? I'm trying to avoid the 'canon' characters due to overuse in earlier playthroughs.

Well you got your tank(Ajantis), your thief and your mage. Coran is a good ranger. And Yeslick can be a good tanker.

Go with a cleric, like Branwen or Faldorn

Take Faldorn. She's the fastest leveling character, you can test the usefulness of high level druid spell in BG1 with her.