I think the best way to achieve something like this would be a number of online servers specific to the purpose, as LFS is really an online game and the AI really are not competetive after you've had LFS for 3 hours (I beat them on my first night in pro mode - then went to demo and got thrashed repeatedly).
The problem with doing this is that people will progress at different rates, and to have different races with different class cars would require more online players.
Perhaps if the servers had car combos at certain times on certain days of the week ie:
Clubman license/server: Monday UF1, Tuesday XFG, Wednesday XRG.
National B license/server: Monday FXO, Tuesday LX4
etc. etc.
Drivers would earn their race license and earn/spend money to buy cars and cover running costs/repairs on their car. All this, more or less, could be done with insim right now.
However to be worth it the servers would need to attract around 50 live players at any given time, in order to fill the different licenses on the server.
The problem with that is as far as 'custom' servers go we've already got 2-3 servers of STCC licensed racing, CLC's custom server, and odd compo club's custom server.
There's 5 servers already attracting large numbers of players to 'something different', in LFS online player terms, that's a significant proportion of players already doing things other than pickup racing.
Just how many custom servers are possible before they start to struggle to attract new racers or start hurting each others online player base?