I'd love to see support for other lj-based services such as InsaneJournal, DeadJournal, GreatestJournal, JournalFen, etc. To be useful to me (and the majority of people I know), it would have to include the ability to differentiate between multiple accounts with the same username at different services.
In addition, the ability to post to multiple accounts simultaneously would be absolutely fantastic.
I second this. In particular, I'd like to see support for Dreamwidth.org, which is presently in its closed beta. The open beta will begin on 4/30. There is some concern over whether any Mac OSX clients can handle Dreamwidth's separation of reading (friends) and access lists. However, iJournal was able to do this. iJournal hasn't been updated in 3+ years though, and I'd love to see xJournal keep getting better and better.
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I, like the_sweet, would also like to see it support Dreamwidth.org since their site allows for cross-posting to LJ and IJ. I don't like having to use iJournal to be able to update my journals. Functionality and ease-of-use for iJournal is no where NEAR where xJournal is.