Regarding Halo 5 sales I think the game will sell 6+ million in 2015 and 10-11m lifetime and maybe more.

The roster is easy, but I feel like picking a consistent art style for the game would be incredibly difficult. They're very different-looking games, so I'd imagine they'd have to find a balance between the more realistic and the more cartoony-looking characters.

Same applies for a lot of games like this though; Smash Bros. being a good example. There will have to be some middle ground drawn though to make sure no characters look out of place though, I agree.

Yes the game is going to solid. Master Chief driving warthog or tank, marcus driving gears based vehicle that big one in gears 2, Max driving a kart made out of crayon, Steve driving a pixelated car, Fable driving some ancient looking car. Man even Rare's characters will be enough to make a game. Banjo, Conker and character from killer instinct. If ms wants to do they can it will also give forza franchise a 3 year cycle since 1 year forza motorsport, horizon, kart racer and then again

This is the canceled rare game



Im showing you this so you see that redesinging characters to match artstyles is not difficult.  Think of masterchief looking like this



or Marcus Fenix like this



Anyways the art could go any different way is my point, not to look like these exactly

Jay once posted picture for few characters sharing same artstyle cartoonish look to it, have you seen the concept image for Rare racer, it is not that difficult. Well I agree a line needs to be drawn between the design so compromises will be made for sure.

Yeah I could see them going that route. I'd like that type of style honestly, except maybe the weird little GoW chibi thing at the bottom. xD

Some of those characters are good but chracters like Arbiter and Cortana wouldn't really fit a kart racer aimed at kids. They meed to create characters that kids would like. They could even give some of those characters their own games like Nintendo does with the characters from Smash Bros. 

I want a real whacky races inspired kart racer with MS mascots. Long tracks, lots of alternate routes, the ability to have time to get out of your vehicle and place traps, change signs (so you fall off a cliff), ect.

^This xD  Also weapons could be interesting.  Plasma Grenades/grenades is the first one that comes to mind as a common offensive weapon