I would love to see it used in more traditional brick&mortar businesses as a booking system where they could change a small fee to get some compensations for no-shows or manage fully their bookings.

Essentially any service business could use LNCal as a booking system.
Seeing the big growth of business that accept bitcoin on https://btcmap.org/ makes me kinda positive about this.

interesting, what do you think will be the first  big non-bitcoin related industry for lncal?

bitcoin

LNCal.com AMA

so far it's mostly people related to the bitcoin ecosystem and some from the wider crypto. it's mostly people that are known in the bitcoin twitter sphere with some outlier more focused on consultancy. This is a function of how widespread Bitcoin Lightning and LNCal are of course. 
The more Bitcoiners demand being paid in Bitcoin the more different industries we'll see joining in these apps.