In my opinion the game should definitiely be modelled after EA's 'Skate' series. Like so:
It should have a similar style storyline, where you play as a customisable character who must complete various challenges and goals (photo ops, filming clips, downhill races, comps, collecting objects scattered around the map) within an open world environment. The completion of these challenges should further drive the plot. There should also be multiple skateparks/pools at various locations, where similar challenges may be situated. Obviously this open-world environment would be littered with street spots, and be accessible in a 'Free Ride' mode of the game as well, just like in the 'Skate' series.
Another important aspect would have to be the scooter customisation, and this would be one of the biggest differences between this game and 'Skate.' I'm not too sure how this would work, like whether the choice of wheels would affect the way the scooter would ride, or whether an Addict Deck would whip faster then say a Proto Orion 5". As long as there is a vast range of products and colour choices (possibly custom graphics?) then it would be one of the best parts of the game for sure. I also think that parts/clothing should have to be bought with money earned from the storyline element of the game, and at the beginning you start with maybe an Ultra pro complete or something and have the choice to buy from a limited range of parts, and various brands get unlocked as you encounter them throughtout the career as well.
I have so much more to say but cbf typing.