Honestly if they are going to spend money on licenses race cars are the only choice. Street cars only if you get a really good deal. But we could at least get a Corvette C7 or Camaro for safety car, the current one needs to be replaced urgently, or at least fix the SLS and bring it to current level of the game...
Hopefully more GTEs to have more variety for league play. Aston Martin given the existing relationship should be easy to add. Maybe Ford GT and a Lambo. Doubt we will ever see a Ferrari due to licensing.
The current LMP 1 formula of hybrids will be phased out in a year or two so S397 has stated they won't focus on those, at this time.