The hulls themselves on the remaining A4s are tired as hell. They need wholesale replacement, not further upgrades. I'm sure the Leo 2 hulls could still find great use as AEVs, ARVs, bridge layers, etc but we need new MBTs something fierce (and at that point you may as well get all the aforementioned tank supporters in the same configuration for logistical simplicity).
If I'm waving my wand we pick up 250-300 top notch M1s with a full complement of new supporters and select reserve units get what's left of the Leopard 2s until the attrite themselves to nothing. Half for training, half for parts. Reservists ran tanks in the past, they can do it now with the right support.