Yeah 2017 sorry. I have 0 expectations for Nadal vs Djokovic at Aus Open.Do you mean 2019 AO, or the 2017 loss against Fed? Nadal won't ever win another set on hard court against Djokovic. It's a terrible match up for him since he slowed down after 2014.
Nadal has been very fortunate with the USO draws in 17/19, perhaps his luck will continue, but the USO does feel more open than any other slam. Wimbledon will work in Djokovic's favour, his serve has greatly improved, next gen are useless and especially on grass. It will be a wasteland soon, Fed/Nadal might have another good run in them but even now Nishikori and RBA made the last QF/SFs...the horror.
I'd say, presuming their bodies don't break down and without the prospect of any comparable talent (i.e. anything close to a young big 3 type), Nadal 2x RG with potential for a USO. Djokovic 2x AO, 1x USO, 2x Wimbledon. Fed's way too old & slow, but may pick up a Halle or Basel title.
US Open is definitely the most open. Wimbledon, any decent grass courters around besides Fed? Lol.