massive fan of the 2 division idea. You could leave it open for countries to decide how many games they want in a series too, and keep tours as a thing (just have a minimum of 3 rule to stop 2 test series).
Particularly good for ODIs as it would allow countries such as Ireland etc to have a shot at becoming a top tier side - they basically already play a lower tier now amongst themselves, just add a couple of current test playing nations with promo/relegation and itd be great.
It'd also allow England Australia and India to legitimately play each other often, as 2 test nations would always be in the lower division - they'd just have to make sure not to get relegated themselves of course.