Mounts and the Master Cycle+

For a sequel to Breath of the Wild I would assume that Nintendo will focus a lot more on the game's mounts. Horses were already an important part of Breath of the Wild and traversing the game's massive world, but the execution was flawed mainly because they lacked the accessibility. You couldn't simply call your horse from almost anywhere at anytime anymore like you could do with Epona in past Zelda games. Nintendo tried to fix this by introducing the Ancient Saddle and the Master Cycle Zero in the Champions' Ballad DLC Pack, but for many players this probably came to late.

The Master Cycle Zero is also accessed only very late in the game, after completing all of its main quests. It helped me quite a lot while searching for all 900 Koroks in Normal Mode, but when I got the bike in Master Mode I was done with pretty much everything else and therefore barely used it at all... For a sequel it would be nice, if Link had the Master Cycle Zero right from the beginning and then received some updates and alternatives for it over the course of the game. But let's discuss this by looking at the different terrains that you'd want to traverse with mounts.

On Land

Here horses are the primary method of transportation with the Master Cycle Zero as an alternative. Horses can be faster, run autonomously and don't need fuel, so they still have some advantages over the bike. But they can also die and need the Ancient Saddle for the same level of availability. So, especially if you would get the Master Cycle Zero Rune early in a sequel, you'd want horses still seem like an attractive alternative or else you won't be using them at all. A good start would be letting them teleport to you without a special saddle, while other saddles and bridles might offer useful abilities.

On Water

This part feels kind of underdeveloped in Breath of the Wild and didn't offer any real mounts yet. You can either swim, which is terribly slow even with the full Zora Armor set, or you can use rafts, which felt slightly inconvenient and more like a physics demo. Let's say a sequel takes place somewhere in the ocean, like on an large island with some smaller islands around it, it would be nice to have actual sail boats similar to the one in the Wind Waker.

Alternatively, there could also be special animals that could be used for wakeboarding with your shield, similar to the Sand Seals in the desert. In fact it could work very similar with the Zora renting these animals to you.

The most convenient thing on water would be having a Boat Rune, however, which works like the Master Cycle Zero and lets you spawn a small motorboat on water. It could work like the motorboat in Skyward Sword, where the Bomb Rune even could be utilized for a cannon and the Magnesis Rune for a salvage tool. It wouldn't be as fast as using the other mounts, however.

Volcanoes

Who wants to travel Death Mountain in Breath of the Wild has to do so on foot. This makes sense, because horses would burn and the Master Cycle Zero probably would explode. The terrain also isn't exactly the simplest. Still, it would have been thinkable to protect horses either with a fireproof potion or with special fireproof horse gear. This would even give them some relevance over the Master Cycle Zero.

Otherwise there could be special mounts around the volcano that live in the hot terrain. Something like a gecko or a special breed of Dodongos (like Dimitri in the Oracle games) could be used to run around volcanoes and even climb steep cliffs. To think of it, there used to be gecko-like Dodongos in Twilight Princess, who even were able to work on ceilings with their sucker toes. Now they only would need to be tamed by Gorons like horses to climb mountains.

In the Desert

For some reason, you can't neither bring horses nor the Master Cycle Zero into the desert in Breath of the Wild, despite the fact that they would technically work. Instead you will need to rent or catch Sand Seals. Catching them has the disadvantage that they will run away, as soon as you let go of them. Rented Sand Seals stay with you, but can't be called from anywhere. So, as a result you often find yourself traversing the desert on foot. Slowly...

One thing they could do is treat Sand Seals more like horses with actual stables, customization and higher availability. The same could go for similar water mounts and even the Gecko-Dodongos around volcanoes, if such creatures were to happen. But it would also make things more convoluted, while they kept it really simple with the Sand Seals in Breath of the Wild. Since they are only useful in the desert, they are just not as important as horses.

One alternative could be making the Master Cycle Zero work in the desert by transforming it into a trike, as soon as it touches sandy grounds. It wouldn't go as fast, but it would work fine on sand. This is where upgrades to the Master Cycle Zero Rune come in, the "Master Cycle+". You could upgrade the Master Cycle for different terrains, which could also apply to water.

Instead of having a separate "Boat Rune", the Master Cycle+ simply could transform into a boat, when you summon it on water or even drive it into the blue. This would keep things simple and convenient, where the Master Cycle would act like a fun transformer for Link's transportation needs.

In the Sky

Sky is the limit for a Zelda game, where in Breath of the Wild you could only glide through the air with the Paraglider, but not fly up. A sequel might change that and even bring back the Loftwings from Skyward Sword. In that game you could only use yours to fly through the sky to drop you off above the lands, but you couldn't really use it on the actual overworld. A sequel to Breath of the Wild might provide more freedom, where you can fly from A to B on a bird creature.

Alternatively there could be special Pegasus-like horses that let you fly. But that would feel really cheesy, where those things are best kept to Fire Emblem. The Zelda series already established the Loftwings and now only has to bring them back in some form.

They could also go as far and apply the Master Cycle+ thing to the air as well. Drive your bike off a cliff and it will turn into a small airplane. Sounds ridiculous? Well, so did having a motor cycle in Zelda and that still worked out great. Of course the main disadvantage of the Master Cycle Zero still applies here, so if it runs out of fuel, you would have to land again for refueling. Loftwings don't need fuel.