Of course, you can suggest. But not in case wherein the requirement is not specific. You can not say until you do not have entire requirement in front of you to analyse. Once done, you can then give your comment about what is the best approach which makes application / project run sucessfully. For.e.g you can suggest most suitable technology to help achieve maximum result, performance and rapid development.
From the design prospective, you can suggest what looks good and what does not. Be in consultation mode not the service provider mode. Everybody is proivding services, but few are actually giving client more of what they are looking for, and that is what add value to their projects, and help you win projects.
Nevertheless, its all upto us how we approach client in such scenario.