I can think of two examples from Denmark:
A group of young zoologists/biologists intended to open a zoo with a focus on Oceanian species in Aarhus - Denmark's second-largest city - back in 2007 or so (Aarhus is the only one of Denmark's major cities to not have some kind of zoo or aquarium, even though it used to have a small zoo that closed in the 1960's). In the news article I read, the municipality claimed that they considered the project to be plausible, but in the news video I saw, these people just spent most the time joking around with each other instead of giving proper answers to the interviewer's questions and didn't seem all that serious about opening a zoo. After the initial news articles, nothing was ever heard of the project again.
A much more serious project was "Tropebyen Slagelse" ("The tropical town of Slagelse") which was a big tropical zoo - similar in size to Randers Regnskov, I think, but with one huge dome instead of three - meant to open near the town of Slagelse. It was clear that the people behind this project were passionate and serious about opening the zoo as they choosed a favorable location, hired architects, opened a fancy website, talked to other zoos, hired a "development" team, did all sorts of fundraising campaigns etc. The project started in 2006 or so, and the zoo was meant to open around 2016/2017. Until 2012 or so, there would regularly be progress, but then nothing was heard from the project in a few years. In 2015, a press statement was sent out, mentioning that the people behind the project deemed it too unrealistic to raise money for such a huge project, and the project stopped.