Between the M1 and the K2 I see the Leo2 club starting to shrink to 3rd world countries.
I actually like what I see in the glossy brochures for the Panther KF51 (or as I call it the PzKpfw V v2.0 - It's namesake was, IMHO, a far superior tank to its heavier and more illustrious Tiger PzKpfw VI counterpart).
Using the relatively dependable Leo2 chassis and a new turret with new concepts makes it a good future system. OTOH the M1 and K2 series are here now. It may be hard for Rheinmetall to carve out a niche unless they can work out a really good upgrade path for old Leo2A4s to KF51s.