He doesn't need "a lack of competition" to win Majors outside clay. He was close to defeat Djokovic (the biggest possible rival) on indoor conditions at Wimbledon 2018. And he had a typical knee injury (mainly because of the draw from hell) at the US Open 2018.
He needs a normally difficult draw (2 tough players but not 4 like the US Open with Khachanov, Thiem, Del Potro and then potentially Djokovic). Djokovic only faced 1 tough player at the US Open 2018 (Del Potro).
Going on his results in the last 8 years and the fact he's getting older, I think he needs a lack of competition and by that I mean a lack of Djokovic or 2017 form Federer. Against most other players he is the favourite so if Fed is not in 2017 form and Djokovic is not there then Nadal still is the favourite at any slam if in good form.
Definitely Thiem, Delpo and Djokovic is hard but you'd expect to play 2 tough players and the lack of this recently is what I mean by lack of competition. A few years back you'd say beating 2 of Fed/Nadal/Djokovic/Murray/Stan/Delpo was normal. He had 1 big 4 player and two sometimes tough players, too much is being made over Khachanov who is promising, but let's not act like he's a proven threat in slams just because he ended up in Nadal's draw.
I know he was close to beating Djokovic at Wimbledon but then again Fed was close to beating Nadal in Rome 2006 and he never got anywhere near that close again. Just because he had a good wimbledon run doesn't mean I see him having that again and again as the years advance. I mean maybe he will next year but if not then next time he's 34 and it likely gets harder.
Injuries are common for Nadal so that's in fact another reason why multiple non RG wins are a bit hard to put money on. Before the USO started I said in a few posts that most times he wins a non RG slam he either doesn't play the following year or has an injury in a match he loses (now 4 of the 6 times he's won a non RG slam) So these injuries don't increase his chances.
But like I said, if he comes into an event playing well and healthy and Djokovic doesn't beat him, then he's got a good shot