The battle over tougher rules for "systemically important" banks took a new turn this week with the revelation that global regulators are now considering a more "flexible" approach.The new approach continues to leave open the possibility that Australian banks could be affected by the global rules for "systemically important financial institutions", or SIFIs.The SIFIs affected by the global rules would have to meet higher prudential standards than other institutions, including tougher capital requirements.In a speech in Switzerland, on Wednesday, the general secretary of the Basel Committee, Stefan Walter, said the committee had developed a methodology that could allow for "differentiated treatment" of banks "without needing to specify a fixed list" of SIFIs. The methodology took into account the five key components of systemic importance, he said: size, interconnectedness, substitutability, global activity and complexity.Walter did not spell out how a system of "differentiated treatment" would work, nor did he suggest the Basel Committee has decided the final shape of its approach to SIFIs.And a Financial Stability Board statement, issued after a meeting on Tuesday, confirmed that global regulators don't yet have a final SIFIs list - it referred only to "progress in work to identify global SIFIs".The Basel FSB statement also suggested the SIFI process is dragging on. Previously, regulators have talked of publishing a SIFIs list by mid-year; the statement said only that the FSB had agreed "to deliver the G-SIFI recommendations to the G20 Summit in November".All these signs point to continuing intense debate on SIFIs among global regulators. The regulators are under pressure from big global banks such as HSBC and Deutsche Bank, which don't want to be singled out for tougher treatment than their smaller rivals. These banks would prefer that any list of SIFIs be as long as possible - HSBC has suggested a list of 80 SIFIs.Australian officials confirmed last month that they are concerned that, at its widest, the list of SIFIs affected by the global rules could include Australia's Big Four. This week's developments suggest that possibility remains.