Please, please, allow multiple databases per application.

I think this is a must-have feature. For example, for Drupal sites, this would enable the "multisite" capability. In this configuration, multiple sites share the same Drupal core code and modules. This makes updating Drupal core and modules a snap, as you can update multiple sites in one step.
Think of a web agency that maintains dozens of Drupal sites - updating one at a time is a lot of work. Drupal doesn't support automatic updates like WordPress but it's rumoured to be available in Drupal 10. The multisite features requires multiple databases - one for each "sub-site".