I find that it is far to easy to enter unbalanced transactions: GnuCash happily and silently creates an Imbalance-USD account to reconcile the imbalance.  Given that this is a (user) behavior that pretty much runs counter to the idea of double entry accounting, it should be much more difficult to enter such transactions.  Consequently, I'd suggest two enhancements:

1. Have a user preference that allows the user to specify that unbalanced transactions should not be accepted.  If the user attempts to record an unbalanced transaction, I would imagine a dialog box popping up allowing the user either (i) to continue editing the transaction (to resolve the imbalance) or (ii) to delete (if the transaction is a new transaction) or to revert (if the imbalance was created by editing a transaction) the transaction in question.

2. At the moment when the user asks GnuCash to record an unbalanced transaction, present a dialog that asks the user to confirm the automatic creation of an Imbalance-XXX split line, and possibly a selection box to specify an alternate account for the residual value (if it's not deemed too cumbersome in the UI).

My personal preference is for the first feature (or both!), but either would reinforce the fact that GnuCash is at its core a double entry system.