Rumours about Saudia starting BHX-JED were circling a long while before they started. I for one was sceptical about these rumours but it happened and the service is performing very well although I have no idea how much traffic is for Saudi itself or for onward connections. And cargo is a factor too
Saudi Arabia has huge ambition to become a major tourist destination on the global stage and is stopping at nothing to do so. So I'd be highly surprised if Saudia don't add another UK route or if Riyadh Air join the party. With Saudia now serving LGW, LHR, BHX and MAN it makes sense their next foray into the UK market will be Scotland and I'd put my money on EDI.
Having said that, Saudia did put ACL slot applications in for BHX so if as stated there aren't any in for EDI, then perhaps it's not happening - for now.
I personally doubt that NCL will see a service to Saudi Arabia ahead of EDI just because Saudia sponsor NUFC.