I REALLY HOPE BESTHESDA READS THIS AND TAKES IT INTO CONSIDERATION
What I felt really strong about while playing Skyrim
1. Make More Large Monsters!!!
The only large monsters in Skyrim were dragons, mammoths and giants. Other smaller (such as trolls) were weak and didn't take long to kill. More huge monsters would be AWESOME.
Maybe these new large monsters should take into account the environment; I felt the water/sea in Skyrim was rather bland, so a KRAKEN (like Kayran in Witcher 2) in deep waters or a HYDRA would be cool. For this to work, Bethesda will have to add a water combat too. (see point 3)
2. Make More Mounts!!!
Other than horses, make something like dragon mounts after capturing a live one, sabre cat mounts (that would totally rule), hugh bird/Griffin riding etc. Horses are boring.
3. Make Combat in Different Situations!!
Mounted fighting (ie on a horse), underwater/in-water combat
4.Make the Player able to FLY!!
As mentioned in point 2, to have a dragon or a bird mount the player must be able to fly!! Being able to fly (either by riding a flying mount or by fly magic) would be a totally awesome addition to TES.
I would be so happy if these were implemented in the next TES game.