Hello Italkian friend, Salutations from Brazil! Well, you can also say: Everyone wants to know how the park will look like! Cheers!

As a sentence, 'Everybody is looking forward to seeing how the new park will be' is grammatically OK, but the problem is that it's a word-for-word translation from Spanish, and 'how xx will be' is not a correct translation of 'cómo será' in this context. You can say this in one of three ways: 'Everybody is looking forward to seeing what the new park will be like.' 'Everybody is looking forward to seeing how the new park will look.' 'Everybody is looking forward to seeing what the new park will look like.' Of these, the first is most natural. Note that you can't combine these sentences at random. It is not correct to say 'how the park will look like', for example. By the way, well done for getting the construction with 'looking forward to seeing' right. Full marks for correct use of a gerund following 'to'!

It is most likely that I would say "what it will look like".