Track limit infridgements have been a problem in European single seater racing for a little while now. I'm glad the stewards have at least decided to open an investigation. No matter the results, it at least gives a small signal to everyone.
Anyway, I'd say just put a good old gravel trap there, and it will kill all sort of controversy... I understand that it would represent a safety issue in many corners, but come on, this incident happened at the exit of a 60 kph hairpin.