Here's my wishlist - A long post, doesn't mean Forza 5 is terrible though
Cars:
After DLC we've got 316 cars (according to ManteoMax) with two confirmed coming with the Infiniti Car Pack, taking us to 318. I think Turn 10 may have one more car pack in them, maybe in September or in October to be a co-release with Horizon 2's first DLC, bringing us to 328 cars. I think Turn 10 have done a great job with the car list, whilst being one of their smallest in their game history, they've managed to encompass so many different styles, manufacturers and types of car that it really does show that despite only having a half your car list than the previous game, it can still be just as diverse.
However Turn 10 do realise they copped some MAJOR backlash for only having 200 cars at launch, even now for only finishing with 320ish cars, despite having their cars re-built and modelled for next gen. I expect them do go all out, and try bring the list in FM6 to hopefully 425+, adding atleast 100 cars from now until whenever Forza 6 comes out and obviously not getting rid of any cars from FM5. Hopefully they do maybe a 60/40 or even a 65/35 split meaning that 65 of those 100 cars are new to the franchise and 35 are ones from FM4 or FH2. Obviously I could name a million new and old cars that I'd like to see, but here are my top 5.
Returning From FM4
2011 Maserati Quattroporte Sport GT S - Loved this deceptive performance, and Maserati are poorly represented in F5
2010 Pagani Zonda R - Can't believe this still isn't in the game.
2004 Vauxhall VX220 Turbo - Loved this car since I first used it in Forza 2, great lightweight car for beginners.
2011 Lamborghini Sesto Elemento - Still can't believe this isn't in also.
1997 Lexus SC300 - One of my favourite drift cars from FM4, makes sense to have it in 5.
New to FM5
2013 Alfa Romeo 4C - Been wanting to see this in a game since NFS MW:2012. Deserves the Forzavista treatment.
1979 BMW M1 Procar - Loved it in PGR4, would be awesome to see it in F6
1981 Ford Fiesta XR2 - Classic. Wished it had been in Forza since the first one.
1988 Mclaren MP4/4 - Too add to the growing collection of classic F1 cars in F5.
1994 Renault Espace F1 - MPV with an F1 Engine in it - Nuff said.
Tracks:
After DLC, we now have 17 tracks in the game, which is an improvement, but more or less where a lot of people agree the game is lacking, including me. Long Beach, Road America and Nurburgring were all great additions, but definitely more is needed, and if I was Turn 10, I'd focus more on getting more tracks ready for Forza 6 at launch, even if that is at the expense of say 30-40 cars. Obviously they have some great omissions from previous games (not just F4) such as Infineon, Hockenheim, Fujimi Kaido etc,etc and tracks that have been continuously requested such as COTA, Limerock, Monza, Monaco etc. I think Turn 10 should look to atleast add 6 tracks minimum. Between Forza 3 and Forza 4 they added 5 tracks, and hopefully they should try to push for 6 to 8 tracks, with a mixture of new and old Forza tracks. If I could choose then this is how it would look.
Returning Tracks:
Tsukuba Circuit
Suzuka Circuit
Infineon Raceway
Hockenheim
Fujimi Kaido
New to Forza tracks:
Monaco GP
COTA
Ebisu (maybe as a DLC)
Obligatory virtual track, maybe London?
Features:
In terms of features, Forza 5 had a few improvements such as Drivatar, (needs tweaks) Handling and Physics (pretty good), Open Wheel Cars (finally!) and Graphics (really good) but didn't really innovate in ways that people thought it would. Now that Turn 10 have had more time with the Xbone, and with Forza Horizon 2's full day night cycle and weather system, lets hope that Turn 10 can implement one of these features. Whether that be all tracks with a night and day and morning lighting configuration then so be it, this would be extremely beneficial, answer many of Forza's critics and add to the games replay-ability.
I think what made Forza 5 lack for me was partly the lack of community features from predecessor games. I'm talking about Car Clubs, Auction House and User-Created Lobbies, three features that made Forza what it is today. These need to be put back in definitely as they are sorely missed for a game with an influx of new community members which needs a place for people to hang out and meet friends. FM5 definitely feels like the most community lacking Forza game in the series, due to the lack of these features as well as it being a launch title and subsequently rushed etc.
In terms of smaller things, more car customization is at the forefront of my list,
with things such as new rims, more engine swaps, more body kits, widebodies, exposed intercoolers, rear race diffusers (where did they go after FM2) and tires are needed to spice up the customization that barely changed from F4. An update to the replays and photomode also is dearly needed.
So overall Turn 10 have their hands full with stuff they need to update and add to the next motorsport title. Hopefully they've learned from their mistakes from F5 and can improve on the engine for F6, and remove any doubt from people skeptical making the jump to next-gen.