In terms of the combat, Molyneux told IGN he'd "love to see brutal, visceral and fluid combat system that left permanent scars. Weapons like short swords, long swords, daggers, spears, long bows and crossbows all having a there own combat feel and specialties."
Molyneux also thinks the magic system should get a complete overhauled, giving players the "ability to 'craft' your own unique system." We pretty much blanket agree with a magic system overhaul if only because it's been so long since we last saw a game in the series, and the way gamers approach combat and magic in 2018 is much different than 2010.
Molyneux would also like for players to be able choose their familiar and have their own horse they can train, as well as their own home they can "build from scratch up to an abode fit for a hero." Which basically amounts to the customization options of pretty much every major budget current-gen RPG. Also, most of those features were available in all of the previous games, which means they're likely to see a reappearance in Fable 4.
Microsoft has been teasing a new Fable game since May of 2017 as Phil Spencer has been a champion of the cause.