A thought I've had is we should ignore leap seconds until the difference between UTC and timezone reference times (e.g. mean midnight at the Greenwich meridian) reaches some significant threshold, e.g. 5 minutes. At this point, all timezones worldwide should perform a coordinated shift by the threshold amount - so Europe/London moves from UTC+0000 to UTC+0005, Eastern Standard Time moves from UTC-0500 to UTC-0455, etc... and stay there until the difference reaches the threshold again (one way or another). We should get at least a couple of decades of forewarning if such a shift is likely to be needed, which should be enough to plan for and test the first migration.

Whether this co-ordination involves everyone switching at the exact same instant, or each timezone switching at e.g. 01:30 in that timezone, is left as an exercise for the reader.

Also... it pushes the first change far enough in the future that the people designing the rules likely won't have to worry about having to implement them. Which might make it easier to agree on them ;-)