[Alexander Persil]

Mandatory pit stops for certain series (GT, formula) would really add some strategy to the racing and spice things up. Even for 20 min races, adding a pit stop would make things much more interesting. You could always increase tire wear/fuel consumption to 200% to compensate for the shorter races. It would certainly add another element to the race, maybe mix things up a bit, and add excitement. 

[Pasha Paterson]

There are a few things you can do with 20-minute series that force a bit of strategy without messing with wear rates or forcing an artificial pit stop.
The MAD Formula MFTC3 can't always make it 20 minutes on fuel at certain tracks, so those series introduce a lot of interesting decisions — "Should I pit or save fuel?" etc.
GTE cars can have super-soft tires, which are fine for qualifying on short tracks but terrible for long tracks or full stints. You could have a 20-minute sprint series with open setup except leaving super-softs as the only tire option. Now you have a strategy choice — Do you tiptoe around and baby the tires, or go all-out and make a pit stop?
I daresay, though, that part of the attraction for some players of the 20-minute races is that you *don't* have to deal with pit strategy.

[Alexander Persil]

Those are some good ideas Pasha. I just like the idea of having more strategic options, to make the race more interesting. Having the option to undercut or overcut, dealing with traffic after coming back out from the pits, those are all things which add more elements to the gameplay.
In F1, for example, pit stops sometimes result in the the most exciting moments of the race.
Just some food for thought

[Handy Hanafiah]

Just to add something from what Pasha said.
The car that requires fuel strategy that I know
-Legends Ford (Fuel tank too small)
-Mitsubishi Lancer Evo Time Attack (boost management)