I think UMK9 would be a great way to celebrate MK's 20th Anniversary. It should probably be released in both formats - DLC for those who have the original MK9, and a new disc-based edition to try and entice newcomers to the franchise.
For me personally, all they'd need to do is add Motaro, and I'd buy it.
One thing, though. I think NRS should "dig themselves out" of the story threads remaining at the end of MK9's Story Mode. I really enjoyed MK9's story, but it finished with a concerning situation - many of the franchise's most popular characters are dead, and they've headlined Shinnok as a big influence in the future.
The story doesn't have to be continued in the next game, but I don't think these particular story threads should play into MK10 at all. The franchise should keep its most popular characters, and I think MK10's story should go in a completely new direction with a new threat to face. It shouldn't be Shinnok - he's a good fighter, but he's no boss.
NRS could keep honouring the arcade classics in MK9, and include MK4 as "Act 4" of Story Mode. Introducing the remaining characters from that game, and telling the story of the battle against Shinnok. This way, Shinnok's threat would be over, the dead fighters may be restored (hopefully with Sub-Zero being human again), and MK10 can be a totally different story.