Characters / Packs from the Xbox version carry across to the Win10 version, I doubt it would but unless that happens on the Steam version, I feel they would lose potential players as well as divide the player base.
I dunno, my friend that I've played tons of KI with sold his Xbone awhile back and has also shown me how terrible the MS Xbox game app is. I could see the Steam version easily getting its own player base. The Steam/PC version of Tekken 7 has turned out to be the best in that case. This just seems like a good thing to me.
I think Iron Galaxy said that the Steam version is coming either late this year or early next year. Steam users will be able to purchase the Definitive Edition (DE) with no option to buy individual parts. Might require Windows 10 but there hasn't been any announce yet of system requirements.
I don't foresee cross-buy working out because Steam purchases not being connected to Xbox/WIN10 store's purchase history. Game saves will probably sync. I doubt the online communities will be different though. I'm willing to bet everyone will play together.
Also DE and Season 3 Ultra edition owners are getting the 3 extra characters for free when the Steam version comes out. I'm pretty happy about that even though I probably won't learn to play those characters.
It's also nice that Eagle is the free rotating character this week!
EDIT: If you're an Xbox Live Gold member, the Definitive Edition is $24 during the current sale! It's also cross-buy, cross-play with Windows 10.
"Challenging my unit was both foolish and reckless! You are nothing more than my prey... one that is soon to be retired!"