Every entered transaction should have to be balanced
In professional accounting there's no reason for being able to enter a transaction that is not balancing. The standard behaviour of Gnucash should be that a transaction cannot be recorded until it balances.
If the user is not sure how to account a transaction, it's better if the user themselve (sic!) accounts the transaction against a temporary "unbalanced" account. Having Gnucash doing so automatically makes the accounting much more error prone.
John R. Sowden commented
A warning window with an option to either save the imbalance to the imbalance account (because you might no know how to solve it now), or return to the transaction because it was an oversight, wold b an option.
Aaron Laws commented
And how should it handle import from file?
Randy Orrison commented
I never want to enter a transaction with an imbalance. I'd prefer to delete the Imbalance accounts and have GnuCash prevent entering a transaction that's not balanced.