They could bring back HMs. However, I don't want HMs to be mandatory to adventure. If they are, well... They could always have HMs be a passive ability. Upon beating 'X' gym leader, pokémon with 'X' Passive ability can go past 'X' obstacle.
An example of what this would look like...You beat Roxanne in Hoenn, and now you can cut trees down. Yeah! Cut trees down. Not helping the envirement, but you can now access new areas. Later on you beat Norman. Now you can surf! Cause you just ate, and it hasn't been thirty minutes yet, so I wouldn't wanna go swimming just yet. Just use surf bro.
Some pokmon will have multiple passive abilities... swampert can surf, waterfall, whirlpool, and dive. So it would have all four of those passive abilities. You don't have to teach your swampert all those stupid moves you'll never use. Surf was actually a really useful move... but there are plenty of normal/physical type moves I'd rather use than cut. Quick attack or double edge(if i'm using aggron) come to mind.
-alternatives-
Flash? I'd love to see that replaced with a lantern or some fancy night vision goggles or something. The lantern would be a heck ton easier to get than night vision goggles so lets go with lantern instead. Much cheaper too. All caves should really be dark. Why is it that I can see plain as day in one cave, but not the other? Where THE HELL is the logic there game freak? sorry for my language...
swim? swimming lessons. swimming trunks. Those little flipper shoes divers wear? Maybe... I like the idea that the player can swim... you know... like EVERYONE ELSE. Your the only trainer in existence that needs your pokémon to take you places cause your weak pathetic muscles are unable to help you swim. You can ride a bicycle and run literal miles without need of rest. I know this is purely game mechanics. The game wants you to utilize your pokémon to the fullest. I completely understand this. I still think its stupid.
I always imagine my character has a godly irrational fear of swimming. He can barely stand even being near it. It takes immense willpower for him to even sit atop his pokémon as it does all the work of taking his dumb a** where he needs to go. Doesn't even thank his pokémon either. When he gets to land, just puts it back in its pokéball. Super ungrateful btw. That's enough ranting though...
waterfall? Yeah... There is no way your worthless aquaphobia having a** protagonist is gonna get his lazy butt up a waterfall. Just go around the waterfall. Or fly over the stupid thing. stupid. You can use your pokémon to fly from city to city, but you can't use it to fly over obstacles. Actually. Just get rid of every HM and keep fly. Fly is the only HM you will ever need. Tree in your way? fly over it. rock in your way? fly over it. waterfall in your way? fly over it. those stupid obstacles that require an acro/mach bike? God I hate those so much... Use fly dangit! Literally every HM can be replaced with fly. The entire game would be so much shorter if you could just fly. Maxie is atop the volcano doing stupid shhh? Just fly atop the volcano, and push him in. Archie in some island secret base doing more stupid stuff? Just fly in there, and push him in the water. There are probably sharpedo in that water...
I wanna end those post on a less dark note. I can't really think of anything for rock smash or strength, or defog.
Rock climbing is a thing people can do... I doubt the protagonist will ever rock climb though. pokémon protagonsits are pretty contempt with letting their pokémon do everything for them. I wouldn't be surprised if Red had his charizard chew his food for him, or his lapras shoot water into his mouth so he doesn't have to pour himself a glass of water. razzle frazzle *walks out of room ,and slams door*
I wanna take the oppertunity to say, I love surfing in alola. It's really creative. Technically, your still riding atop a pokémon. Not an actual surf board, thus bringing me back to my previously mentioned point of the player being over reliant on his pokémon. However I wrote too much on that subject already, so im going to stop poking the dead horse, and just stop this post right here, before I continue ranting again.