My ideas for F4:
Chronologically F4 is set later than F3, but...
Theresa uses the power of the Spire to blast the Kingdom, a sacrifice to permanently end the darkness. History repeats itself, it turns out!
F4 is thus a return to low-tech and is set many years after 'the fall'.
No main story, just a huge sandbox world in which you try to survive as an adventurer/merc, starting as an outcast teenager.
Patrons who want things done/found/stolen/captured/killed.
Tombs, towns, dungeons, ruins, caves, camps.
HUGE world, many 'zones', loads and loads of places to see/things to do.
No dog.
Climbing/rope use to allow truly 3d exploring.
No more being blocked by knee-high scenery.
Logical loot types and placement. Randomised to make game hugely replayable.
The ability for a character to die, and for each character to be different.
Augments, not morphing.
Combat style, graphics and other engine mechanics are simply tweaks of F3.
The need for your character to eat, drink and sleep.
New and old baddies, no lazy re-skins. Baddies do NOT level with you.
WOW factor scenery.
Think of it as 'Fable returns to RPG and meets Oblivion/Fallout/TW2'. Variety and massive scale would be the key - sandboxes need to be both big and detailed - a tough ask.
I dream of this game. Will I ever see it? Sadly, I doubt it. TW2 is looking tasty, though I much prefer the Fable graphical style to anything else around.
Whatever form it takes, F4 needs to be vastly bigger and longer than F3.