Okay, so I'm gonna try my best to figure out what the island trials are going to be (I won't bother with Totem Pokémon for the most part because we still don't know the full Alolan Pokedex yet):
Island 1 (Melemele Island): Normal, Bug, Flying, Rock
All four of these types have historically been early gym leaders--specifically, they have all been a past region's first gym specialty at least once in the past. As such, all four of these Trials are probably going to be fairly easy and straightforward.
Flying: A tutorial-based Trial, showing you the ropes. Probably at the Pokémon Trainer School where you learn the basics, with Pikipek's first evolved form serving as a totem.
Bug: Make your way through the woods. Vikavolt will be the totem Pokémon.
Normal: Since it appears to be beside the jagged a cliff side (possibly near that waterfall), and it's Gumshoes, it's probably going to involve locating a secret spot.
Rock: Find the missing Rockruff. His evolved form will be the Totem Pokémon.
Completing all four trials will net you a fight with Hala, who will use Pokémon of each of the above types.
Island 2: Grass, Water, Electric, Fire
Dealing with the Starter types, here. The trials are clearly going to become tougher than earlier, but still fairly easy to figure out.
Grass: Mallow's challenge is a scavenger hunt, and Lurantis is the Totem Pokémon (confirmed).
Water: Lana needs you to surf in the waters and hunt down a specific Pokémon--likely the Totem (The general consensus does appear to be Wishiwashi (Schooling form) as the Totem Pokémon.
Electric: A popular theory is that it's going to be a Totem Alolan Raichu. Nobody knows what Sophocles' trial might be or where, but it might actually be in the lighthouse, and I believe his trial to be a trivia thing.
Fire: Kiawe and his Alolan Marowak will likely challenge you to a dance off, and it will be a Memory Game. Alolan Marowak might be the Totem Pokémon, but I wager it's probably something we haven't even seen yet.
Island 3: Fighting, Ground, Ice, Poison
Here, I'm gonna expect the story to really pick up, as it will involve Team Skull more than ever. We probably would have to deal with their antics before taking on the trials themselves, and the trials on this island will surely push you to your limits.
Fighting: I'm gonna just make the assumption that that giant Pagoda is a trial spot for this type. And this trial will be 100% battle-related as you have to fight your way all the way to the top, where you'll encounter the fighting-type Totem Pokémon.
Ground: That muddy-looking region in the southeast. I'm gonna assume that it will involve an archeological dig in some ancient ruins, and you will need to complete a truly labyrinthian trial and solve the riddles of the place in order to face the Totem Pokémon.
Ice: The top of that mountain. It will probably involve an extremely dangerous hike up the mountain that you would need to complete without any help from a Pokémon Center, and without any quick transportation back. As such, I'm gonna assume that reaching the top of the peak will be the Trial, and based on the description, Alolan Ninetales (or that snowman Pokémon we heard from both of the leaks with any sort of credibility) will likely be the Totem Pokémon here.
Poison: This trial could also potentially be the Dark-type trial, but who can say? Either it will be in that walled off city that is occupied by Team Skull, or that weather institute (that will almost certainly be plot-related). It's kind of hard to pinpoint who the Totem Pokémon is gonna be or what the specific trial is going to be, however, since we know very little, if anything about it.
Island 4: Psychic, Dragon, Fairy, Steel/Ghost
The supernatural trials. An unlike the other three islands, I'm gonna suspect that none of these trials will have trial captains, and that you will have to figure out the trials on your own. And isn't it fitting that the types that one would most affiliate with Legendary Pokémon are gonna be the trials here? As for Steel/Ghost, the most I can assume is that said trial is going to be one and the same in both versions, but the Totem Pokémon will be version-exclusive--Solgaleo for Steel, and Lunala for Ghost.
That just leaves us with two secret Trials (on those two tiny islands not part of the main region): The other typing for Ghost/Steel, and the final one (either Dark or Poison). I'm gonna assume that both secret Totem Pokémon are also going to be Legendary Pokémon, however. The Ghost/Steel-type representing the other version's exclusive's totem will probably be Mashadow, with a unique ability that shapes its appearance to look like either Solgaleo in Moon or Lunala in Sun (depending on if it's holding either the Steel or Ghost Z-Crystal respectively). My theory too is that as secret Totem Pokémon, they don't have any trials at all and you simply have to hunt them down.
The island closest to island 4 is going to be Mashadow's resting place. While the island with the palm tree is probably going to contain another legendary that nobody has any idea what it could be. I suspect on another thread that it's probably going to be a fifth "evil" Tapu Pokémon that was sealed away to keep the Alola region safe, and my initial thoughts might be Dark/Ground or Dark/Fire(maybe a devil Pokémon). But it could be Dark/Poison too now that I think about it; the Dark typing can counter Solgaleo, Lunala, and Mashadow (especially since Steel types apparently no longer resist Dark) while Poison is meant to directly attack the Tapu Pokémon (since they are all part Fairy-type). This could give credence as to why Zygarde--who is native to Kalos--would have such a huge presence in the Alola region as the only thing that could stop this evil Pokémon.