Support for autonomous transactions (i.e. the ability to nest a unit of work) would allow for more concurrency. At the moment an operation that is logically independent such as a logging operation can lock resources until the transaction as a whole succeeds.
A great example of where this would be useful is in the implementation of sequences. Once a sequence range is allocated the change can be committed independent of the surrounding transaction. Doing so allows other transactions to allocate sequence ranges while the first transaction is progressing. This allows for greater concurrency.