UBS says it has long since finished restructuring its investment-banking arm. Behind the scenes, though, the Swiss bank recently toyed with potentially far-reaching alternatives for the unit.

As recently as September, UBS held talks with Japan's Sumitomo Mitsui Banking Corp, a unit of Sumitomo Mitsui Financial Group, about forming a partnership that would have resulted in a significant portion of UBS's Tokyo investment bank being moved off its balance sheet, according to several people familiar with the negotiations. Those discussions appear to have stalled, although one person said they could be rekindled.