There is no reason a discretionary trust cannot be created with a single
object of the discretion. Clearly, though, there needs to be other
beneficiaries, or the potential for other beneficiaries to prevent a
bare trust arising, whether in income or on the trust fund generally.
If there is power to add beneficiaries, which cannot be defeated by the
death of that object, or of the settlor, that will help.
In any event, I would recommend the inclusion of both a power to
accumulate and a fixed default beneficiary (preferably a charity),
rather than having a resulting trust to the settlor's estate in view of
potential tax complications