I think we can probably leave the discussion about Spoons now, it's highly, highly unlikely to ever happen....
For me personally, a "theme" could be related to the menu items being offered, or to the restaurant itself. I'd rather not see fast food franchises back and see Towers do something decent themselves, but even when we had those franchises at least KFC, Pizza Hut and Maccies were generally themed to the areas they were in. They were out of the ordinary, something different from the norm. The massive car on Cred Street McDonalds or the fans on the walls in Katanga KFC. In house, we had Nemesis Nosh - not selling any food specifically themed - but served out of a dystopian double decker bus outside Nemesis. At present those themes have been watered down to either being non existent like the large Burger Kitchen, or a vague vinyl job with a few bits and bobs chucked in like Just Chicken.
More recently we've seen burgers themed to different rides in Rollercoaster Restaurant and we've seen themed menu items for Scarefest. They've done it in the past, so there's no reason they can't do it in future.
Bottom line for me is firstly I'd just like some variety that's of decent quality. I accept it's going to be more expensive than outside of the theme park, but for god's sake sell it to me with some half decent presentation and something that's actually unique.
Secondly, as I've mentioned before a theme park should have food as part of the experience. I want escapism at a theme park - I shouldn't walk into an outlet such as Burger Kitchen and feel like I could be wandering into any random burger place in a shopping centre. Give guests something out of the ordinary, or fantastical as the marketing bods are all too keen to say.
TL;DR - Put some effort and thought into it, it's not too much to ask.