Darkfallen have traditionally been more aligned with the Horde in the form of Dark Rangers. However, there's been discussion about Darkfallen being a neutral allied race, mainly because there is now a precedent for Night Elf Darkfallen (Sira Moonwarden and other night elf archers/wardens raised by Sylvanas in the War of the Thorns during BFA era). Interestingly, according to WoWpedia, Sira Moonwarden is currently in the Stormwind Stockades as a prisoner of the Alliance, while it seems the other night elf Darkfallen have joined the blood elf Darkfallen in following Calia Menethil in the BFA epilogue questline. The foundation for neutral Darkfallen is there, but as someone mentioned earlier in this thread, Blizzard would have to justify them joining the Alliance through some kind of reconciliation with their living night elf kin.
As for Tuskarr and Centaurs, I want 'em. I'm one of those people who gets excited by a potential allied race any time Blizz updates a creature model. And now with the updates to Tuskarr, centaurs, and gnolls... I want 'em all.