So I was actually doing research on some of the dark type moves out there. And a lot of them aren’t as bad Crossroad Killing. Assurance is “Make Sure”. Night Daze is “Night Burst”. Brutal Swing is just Brutal Swing. Feint Attack is “Sneak Attack” Foul Play is called “Trickery” Knock Off is “Knock off” Payback is “Retribution” Power Trip is “Be Audacious” Pursuit is “Pursuing Attack” “Sucker Punch” is Surprise Attack. If they can have Ash say Revenge. I think some of these are fine. Dark pulse is apparently evil pulse in Japan. But Serena’s Pancham knew it
Thanks, Zariful. Either way, like I said, there were other Greninjas that used Night Slash in the anime, so I don't see a problem with Ash using it too.
Now, I don't believe the popularity of a Pokemon really involves in its appearance in the anime or if Ash or any other character will capture it (at least, not entirely), but I have checked the lists of the most popular Pokemon, and I believe SOME THINGS could actually happen. Let's see:
1º) Greninja (1º): Surely, Ash's Greninja will return. Very likely, as it appeared back in the promo poster of the new series.
2º) Lucario (2º): Practically confirmed Ash will have one, after evolving the Riolu that will come out of the egg from the next episode.
3º) Charizard (4º): This Pokemon is really overused in the anime sometimes. But even so, I hope Ash's Charizard will return in one episode.
4º) Eevee (20º): Two of his evolutions have an even higher position; Umbreon (5º) and Sylveon (6º). We can assume that, in case Ash captures one Eevee, it could evolve into one of these. I personally think Umbreon is a better option, because Sylveon really looks too "female" for Ash.
5º) Garchomp (7º): Luckily, Ash's Gible will return and Ash will train it until it evolves. It's very probable, even if we consider his Poke Ride as Ultra Guardian was a Garchomp. (But I still don't get how this Pokemon can Fly in the Anime, but not in the games).
6º) Gengar (10º): Ash already captured this Pokemon.
7º) Tyranitar (12º): I really wish Ash's Larvitar will return somehow, evolved into a Tyranitar and he will officially join Ash's team.
8º) Toxtricity (14º): Not entirely convinced, but this Pokemon could actually work for Ash; especially if Ash gets a Rillaboom too.
9º) Rowlet (16º) and Decidueye (22º): I hope this means Rowlet will return, and considering how close his final evolution is in votes (also in Alola's Pokemon Popularity), it will eventually evolve to Decidueye, giving Ash his second Ghost Type after Gengar.
10º) Luxray (21º): I'm not really convinced, but if they are ever going to give Ash another Electric Type, this could be a good option. He once befriended a Luxio.
11º) Zoroark (23º): This is another good Pokemon for Ash.
12º) Lycanroc (24º): It was already hinted in the new ending his potential return, and I really hope it will be truth.
13º) Corviknight (25º): Another Steel type for Ash, which is also the regional bird; so it's quite probable Ash will get it.
14º) Flygon (26º): Ever since the first Hoenn's movie, this has been seen as a good Pokemon for Ash. I personally like the idea.
15º) Sceptile (28º): I hope Sceptile will return for one of Ash's Pokemon Battles.
16º) Blaziken (29º): There has been some hints about Blaziken in the promo poster and other places. If Ash captures this Pokemon already evolved, it would be a great add to his team.