For one, more Clone Wars nostalgia for fans. Since Skirata ended up actually adopting his Null ARC Troopers, it would be cool to see a fatherly figure to the clones in canon; something that wasn't even really seen in the EU besides in Republic Commando.
Two, Skirata and Vau might be old, but that just means they're experienced. It would take A LOT of convincing, but to see the strategy and leadership that two grizzled Mandalorian warriors could bring to the table would be extraordinary.
Fen Rau is cool, especially since he's voiced by Kevin McKidd (RIP Soap), but he still has much to learn. He's brash and arrogant. If he hadn't heard Gar Saxon actually say he was planning on killing Rau anyway, Rau probably wouldn't have joined the rebellion.
Three, outright hostility to the Jedi. Skirata (and Mandalorians in general) have a well known history of not liking Jedi. Mostly when we've seen hostility to the Jedi in the past, it's been the Sith trying to just kill them. Skirata has a deep hatred for the Jedi, only rivaled by his hatred of the Kaminoans and Death Watch, but doesn't necessarily want to kill them. To see a character say "go away, you're not welcome here" to a Jedi in canon would be interesting.
Four, I really want these characters back.