Yes, if Trump himself is going to deign us with his presence then you'd expect it to be a fairly significant announcement, I tend to doubt he'd come all this way to announce a free minibus tbh!
As Flying.Penguin suggests, I could see him being interested in biz/GA or perhaps even high-end charter flights for pax going to Turnberry (although I'm not sure he'd want them passing through the current terminal!). Perhaps he might even like to build a small private VIP terminal.
I'd agree though that it seems an unnecessary step for him to buy the airport outright. Why take on all the expense and assorted hassles of running an airport when you don't need to? Even if the SG sold it to him for £1 he'd still have to take on/settle the millions in "loans" the airport was given, otherwise the EU could become quite interested!