3D hitboxes to 2D hitboxes is a total rework of the game engine, it is not something you can simply edit or swap out and be done. It also requires a monumental time to test and adjust to make it work. Something a patch can not do.
No, the game allready incorporates 2d hitboxes. Further, if dlc containing alt outfits can change frame data and hitboxes, it's also possible through patching. 2d hitboxes dont take up a lot of space. I think you can quite easily fit al the hitboxdata in a couple of MB's. It's not a very neat sollution, true, if done through patching.
In a nutshell TL;DR for the above quoted things I mention, there is no on/off switch you can simply activate which is patching essentially. Patching is just editing files in the existing game, it isn't adding/changing anything significant to the game.
This makes no sense at all. You can download entire games through store. Code and data and you would also be able to change existing data
I think the biggest change would be in the framedata/hitboxes. This would be really ugly through patching but for most people by itself isn't worth buying a new disc for.
Improving the trainingmode to include to have a recordable dummy? Is nearly already there. The recording you can make now is exactly the same, it records input, nothing more. I had a combo in there that worked in one version and didn't work after a patch. The only thing you need to add to it to make a good training dummy, is mirror inputs when it ends up at the other side and make the input loop.
Spectator in KotH? Not a big change imo. Give idle players a 'sit out' option. Dont even need to be like SSF4, where you have to wait for the host to timeout. And players who are on the phone or whatever can use the option as to not delay other matches.
Save replays? At this point hardly worth the effort with framedata changing all the time. When a stable version is released it is not that much different from recording how its done now.
What I would do is release UMK9 through DLC, make it 20 dollars or something like any other downloadable game and make it check for the mk disc during runtime. You download the entire game. Then release it on disc for 60 orso because the DLC is going to be a couple of GB's.