Like the title says, have Germany win World War One only to fall shortly thereafter, let's say fifteen years.
My scenario, which may be plausible or not, is a second Mexican-American War breaking out sometime in late 1917 when US troops have arrived in some numbers, but not enough to be decisive. The extra troops are of course directed at Mexico due to America coming first. This allows Germany the extra time it would need to grind the Allies to dust while weakening them enough that post-war they're struggling to hold their empire. Over the next few years, Germany is overstretched and collapses either due to revolts in Mittleuropa, a war with the Soviets, or both.
My scenario, which may be plausible or not, is a second Mexican-American War breaking out sometime in late 1917 when US troops have arrived in some numbers, but not enough to be decisive. The extra troops are of course directed at Mexico due to America coming first. This allows Germany the extra time it would need to grind the Allies to dust while weakening them enough that post-war they're struggling to hold their empire. Over the next few years, Germany is overstretched and collapses either due to revolts in Mittleuropa, a war with the Soviets, or both.