A few suggestions.
An Actual RPG Game: What really made me enjoy the Fable games? It's not afraid to rip the hell out of the RPG genre with parodies based upon familiar cliches. The table-top D&D side quest was perhaps my favorite mission in the whole of FIII. More of that please! I don't mind it being RPG lite, but the game has still got to be an RPG an Anti-RPG if you will. FIII (although I consider myself in the minority who still enjoyed the game ) jumped the shark and became more of a casual adventure. I'm not asking for stat-crunching or level grinding but at least stick to the aesthetics of what makes a great RPG fun.
The Hero: So ridiculously overpowered and unbalanced in the game. Skill & Will make Melee practically obsolete. Either make the mobs tougher or add time-out penalties for spamming the same combat method all the time. Combat got too streamlined to the point of absurdity, virtually a game breaker in it's own right.
Considering how much Lionhead pride themselves on choices open to the player, why is the character creation screen so limited and why can't I decide what the starting character should look like other than his or her gender?
NPC's : I love that the general population are easily pleased gibbering idiots, it's actually something that I love about the Fable series, why can't I get them drunk anymore and watch them groan, stumble and vomit because that's what I expected more of? The amount of time I spent in Hook Coast getting that whole village drunk exceeded the amount of time I spent on some other games because I thought it was genuinely hilarious.
I do not want to do Fetch & Courier quests all the time to get in their good books and neither do I want to be given a limited set of options either. The fact that I could only interact with one NPC at a time in FIII kind of ruined what I liked about the game. I enjoyed showing off trophies in the previous games and now renown and experience has been replaced with guild seals which make a lot less sense to me. The bards and the town crier are made redundant now, why?
Main NPCs: Why are they so black and white, why can't they be a little more complex? As much I despise Theresa with her smug vagueness, the one thing I do like about her is how much she makes me question her motives in the entire series like her grabbing The Spire, none of the other main NPCs seem that three dimensional and just serve as cartoonish abstractions in the moral compass.
Other Heroes: Too many times we're reminded of past heroes and past glories, I'm not paying money to play a game reminiscing about how epic life was in a previous era in Albion's history, give me some other heroes to duke it out or team-up with. Lionhead decided to ditch the Guild of Heroes, fair enough, give us an alternative to that infrastructure and not have us running around ruins instead.
The Love Interest: I couldn't care less if FIV has one or not, but at least make the love interest actually interesting if one is included and not just someone that disappears for the majority of the game and give you an STD when you marry him or her. They should also be allowed to be customized with the gifts I give them such as wearing clothes bought for them to stand out from the rest of the identical NPCs in the game.
Legends & Lore: For me this is one of the weakest aspects of the series, there's definitely a bare-bones structure for something truly epic but Lionhead need to address this just as seriously as any of their gameplay features. Right now any reference to past events in previous games such as the old kingdom feel very crude and vague. The fact that some people have taken the time and effort to write their own fan-fiction should speak volumes to Lionhead that folks want a game with a strong sense of historical lore. I completely understand the past gets forgotten as time goes by with each game but come on, why aren't there quests that answer any of these issues other than a vague and tacked on explanation from the likes of Theresa? We have these beautiful regions with ancient ruins and a few books to give the player a little background but it's all basically simplistic, some more solid canonical references would be great. Considering how it was referenced in previous games, why didn't we get Samarkand but we got Aurora instead? The people of Knothole Glade decided they had enough of balverines so they shipped out to an even more dangerous region? The reason why so many folks seem to love Fable: TLC is because Lionhead did make a genuine attempt to flesh out this stuff which the sequels did little to add upon or in some cases added confusion to the mix.
Regions With Content: Lionhead have an incredible talent with shaping beautifully rendered areas, they add a lot of atmosphere which some RPGs fail greatly at. My problem is there is practically nothing else to do there after you finish the main game. There are a few regions with mini-games like The Mercenary Camp or Mourningwood Fort but that's about it.
Landlord Ownership & The Economy: Seriously, the series is turning into a landlord sim more than anything else. The fact that FIII's final ending was based primarily around how much gold you raised and donated to the treasury in order to save the population was farcical and played little with your Hero's physical attributes . For everyone that finished the game months ago, who else is sitting on at least fifty million gold and wondering what to do with it? Why don't populated areas have banks, how great would it have been if you could stop or aid a gang of mercenaries pulling off a heist (a choice that would not have been dissimilar to what was available in the first Fable)?
What I would want is to have the option to buy or even conquer a region and own the land itself. The houses are built by the NPCs and they pay you to live on that land. I really don't care about repairing a property or redecorating it. It would be great to clear a dangerous region infested with Balverines, Hobbes or bandits and then allow either gypsies, farmers or nobles to build their homes on the cleansed land. Each choice would have benefits, e.g. specialist shops available based on the player's decision on which class to occupy the region. You could reclaim Oakvale again since public opinion seem to demand that region to be brought back to it's glory or leave it as the devastated and spooky Wraithmarsh.
Mobs / Enemies & Bosses: There's practically no real learning curve in confronting any of them. I was gutted that Trolls were wiped out but the new minions, shadows and sentinels were at least a refreshing break from the usual mercenary, hobb and balverine library of enemies. Hopefully FIV have an even more abundant range of mobs to fight. Make them lootable or skinable too to shake up the jobs in the next paragraph.
Shops & Professions: Vendors were almost irrelevant in FIII considering how limited and useless the items were. The biggest setback was having to run around in another player's game to get all the other rubbish weapons which should have been available to everyone's game world in the first place. I couldn't give a damn about weapons morphing as it was so poorly thought out in FIII and considering all the screenshots I've seen they're not all that unique. What would have been better was to allow the player to (a) fashion weapons, clothing, personal styles and potions, or (b) as a quest reward based on whatever outcome you have decided.
Fix The Dog OR Ditch It! Initially the idea of having a faithful companion sounded great, but other than digging up junk the dog has been mostly useless. I'd rather have a comical alternative like a parrot, monkey or an Old Kingdom style Monitor powered by Will alone and morphing the same way as the dog did back in FII.
Better Demon Doors: They've slowly become more phased out and underwhelming with each game, especially the rewards, enough with the potions and exp. already, give me fancy weapons like the first game. How about Demon Doors that actually have a lost, forgotten and thriving community living behind it instead of them being artistically rendered dead end hallways.
Chests: With the exception of the character of Chesty and some silver key chests all the other chests are pretty disappointing including some behind demon doors. Nothing really notable at all.
DLC: Don't scrape out actual in-game content and then sell it on as DLC, that's pretty shady. It's also worth noting that the DLC so far for F3 has been largely shallow and cosmetic - selling dyes and outfits is hardly going to win people over. Give the player their money's worth and not some superficial dollhouse nonsense, some actual further quests and adventures that are relevant to the game and ties everything up nicely.
Setting - Time Period: As much as I loved Fable: TLC for it's Medieval setting, it doesn't bother me if Lionhead continue to go further on in the time frame to early 20th century in Fable IV. I can just play TLC again if it bothers me that much. I think Lionhead have backed themselves in the corner in this respect but I've got to admit, I'm looking forward to whatever era they decide upon next. As long as they address some essential game play mechanics and bugs and such, I'll be that sucker and cop the next game regardless.
Apologies if this reads like a rant, I didn't mean it to sound like one, just a passionate fan is all.