From Longman Dictionary of Contemporary Englisha fertile imagination/mind/braina fertile imagination/mind/brainIDEAan imagination, mind etc that is able to produce a lot of interesting and unusual ideas → fertile
Examples from the Corpus
a fertile imagination/mind/brain• Even now no-one seems quite certain whether this was a fact, a half-fact or the product of a fertile imagination.• He is said to have been convivial, widely knowledgeable, with a fertile imagination and a whimsical sense of humour. 