Successor Trustees can be any adult individual or a Corporate Trustee, such as a bank. Normally, people name their most responsible child as their Successor Trustee to take over when the Trustee (you) and the Co-Trustee (your spouse) are both incapacitated or gone.
Most people also name a Second Successor Trustee just in case the First Successor Trustee is unable to serve for some reason.
If you do not have an individual capable of handling the duties of a Successor Trustee, you may want to consider the Corporate Trustee option.