I'm not sure I'd call the bolt on stuff you mention as 'depth', however it does lack game play modes, thankfully the core racing is so good it doesn't matter at all - especially with the online mode. Arguably you could say it's purity is a strength.
It hasn't been confirmed but it stands to reason there will be. Considering how well the first one has sold and how shallow it is ( that's not to say it's not a good game as it is ) there most likely will be a sequel. A lot can be added to the game and hopefully they will have more time when it comes to developing the second one.