Pocket Edition mentioned features
Many features have been mentioned for inclusion in the Pocket Edition.
Note that this page only documents features that were mentioned but are not yet in the game. Features in development versions are described on the parent version page.
Replacement of the Nether Reactor
Johan mentioned on the issue tracker that the Nether Reactor may be replaced in the future. Via the MCPE subreddit - Johan stated that the next iteration of the Nether Reactor will be some kind of "super dungeon" and that the Nether dimension is not currently on their radar.
In reply to a comment on reddit, Tommaso stated he thought about it for 0.9.0. When the topic was brought up in the BlockTalk Q&A, Tommaso stated that he doesn't know how to deal with the Nether yet but that it “shouldn't be that bad in the end… could be doable”. He stated that if the nether is implemented, the “Nether Reactor will do something different to what it does right now” On Tommaso's todo list, the Nether is listed.
Tomasso stated on reddit that he would like to add a "difficulty" number that mobs could have depending on their circumstances, such as resource that it spawns from and weather. This would vary the amount of hostile mobs at night and the amount of recources gained from each biome. He also stated that he would like more biome-specific mobs as well as biomes with specific difficulty/mob distributions.
Other world sizes
Tomasso stated on reddit that there may be other limited world sizes in addition to the current "Old" world type (256x256). These world sizes could be 1000x1000 and 10000x10000. When Realms is re-added, it may have a limit of 10000x10000. 
From the PC version
The developers have mentioned many features from the PC version which will eventually be included in the Pocket Edition.
- Ender chests
- Bonus chests
- Item Frames
- Riding pigs
- New sound engine, note blocks , jukeboxes and music discs
- Varied minecart types and redstone-related blocks and items - should be this year
- Mobs AI, like the PC version
- 256-block height limit
- More difficulty modes and Hardcore mode
- Stained Glass
- Texture pack support
- Twitch streaming
- Survival interfaces in Creative
- Sharing worlds
- Server plugins
- Controller support
- Splash screen for Android devices
- Improved Controls with sprinting and crouching, relocated jump button and large device support
- Possibly an animation for switching items
- Android will have updated sounds
- Creative Inventory, like the PC version.
- New Buttons:
- Breed [Animals/NPCS]
- Dye [Sheep]
- A version exclusive underworld underground structure
- Cave Culling 2.0
- Change of GUI button texture
- Multiplayer chat scrolling
- Demo/lite version re-release
- External devices support
- Programming interface
- Copy worlds
- Tommaso mentioned adding 3-D grass on grass blocks; although he decided against it as it was expensive and clashes with the style.
- Windows 10 Edition only structure - seen in demonstrations of the Windows 10 Edition at MineCon.
- Issue tracker: MCPE-607
- http://www.reddit.com/r/Minecraft/comments/terrain gen/cbq67hj?context=1
- http://www.reddit.com/r/Minecraft/comments/terrain gen/cbq67jm?context=1