Now, whilst Apple has conceded that the on-screen keyboard isn't quite up to scratch for enough people that it warrants proper external keyboard support, I think it's highly unlikely they're going to extend that to mouse support. Think about the 'Pro' moniker. Professional artists will use the Pencil (plus touch control with their fingers). Professional writers will use the keyboard. Neither group has a huge need for a mouse - if they do, they certainly don't seem to be shouting about it at the moment.
So the vast majority of pros, doing work, aren't looking for mouse support. Casual consumers (there's a chance that Apple is phasing out all iPads except the iPad Pro which, to be honest, makes the name seem a little unwarranted, but that's another story) are best served by the touch screen, and are using tablets in their hands, not placed down on a surface, and certainly not on a surface with enough room for a mouse.
Even if a pointing device were required, I'd argue against a mouse for the table form. Mice require certain surfaces to work properly, if at all. They either come with an annoying wire, or an annoying requirement to source and charge batteries (although the latest overpriced offerings from Apple are trying to deal with that). They sometimes sort-of support the excellent swipe controls that trackpads have nowadays, but rarely, and they're not as good. Mice are pretty bad, and are probably nearing the end of their ubiquity. If Apple really were to support a pointing device (which, as another poster points out, would require software changes that I'm not even going to begin to address), they should go with a trackpad or a trackpoint on the Smart Keyboard.
Having said all of that, I clearly don't understand what your use-case actually is. What do you want to use a tablet for that requires mouse support?
Because iOS apps would have to be revised to support mouse clicks, scroll wheels, mouse overs, contextual menus, etc. Or maybe iOS would just treat the mouse as a single finger, limiting its usefulness.
A mouse or equivalent would be useful for certain things, such as VNC, emulators, and testing desktop versions of web pages.