Help the dramatic architect complain about a ruined masterpiece.

The grand glass pavilion ________ the crown jewel of our city's skyline, but unfortunately, the mayor abruptly canceled the funding last year.

The correct answer is was to have been.

To express an unfulfilled plan or arrangement in the past, we can use the quasi-modal structure was/were to have + past participle. It elegantly shows that something was officially planned but never actually happened.

"Must have been" expresses a logical deduction about something that did happen, which contradicts the mayor canceling the funding.