HM's are usually to clear your path (Cut, Rock Smash, Strength), or make it easier to continue your path (Flash) or even possible at all (Surf, Waterfall).
I like the idea of Whirlpool returning, I like the move. In general; the trapping moves should be more present. They were pretty overused in G/S if you ask me, but they were pretty much gone after that (they were there, but very few pokémon had them).
However, I really don't see use of a new one right now. They could add stuff like landslides or whatever blocking your path, but they already got 3 kinds of path-clearers, adding more would be overkill imho. Maybe they could add pseudo-HM's (Secret Power etc.) too change the types of the pokémon you encounter. Like Sunny Day making you encounter more (more, not
just) fire pokémon and Rain Dance making you encounter more water pokémon.
Also, it would be nice if the pokémon summary gives a little bit more information, like the size of the pokémon, what moves it had (
HAD!) and maybe some other misc. info about the poke (yeah, I love info, I always read the pokédex too

).
EDIT: Oh lol, I see other people
don't like the idea of whirlpool
