Hi, I'd like to help you with your question.
It probably is a good idea to let your daughter in law to be plan her own wedding. Although you may not feel it is appropriate to have a potluck wedding, telling your daughter in law your opinion may start you off on the wrong foot with your relationship. She is going to feel defensive and probably think twice before coming to you with any questions or problems.
You can, however, offer to help her with the wedding. If she says yes, make some suggestions. Be careful how you do this, however. You want to word it so your ideas come out as helpful, non threatening suggestions and not "my way is better" ideas. A good way to do this is to say, "What do you think of this idea, or this idea?" That allows her to say yes or no or pick which one she likes, all without you imposing your ideas on her. Ask her how you can help as well. She may have lots of questions and depending on her relationship with her mother or other female relatives, she may have no one to talk to about her wedding. An experienced mom in law can calm a lot of fears.
Depending on how involved your son is in the wedding preparations, you could also talk with him about your suggestions. But be careful here as well. You do not want him going to his bride and telling her that you are unhappy and think their wedding is embarrassing. Talk to him as you would your daughter in law to be. He may be more receptive, however, since he is your son.
Wanting to develop a good relationship with your daughter in law is wonderful. You are making an effort and that in and of itself is a good start to a good relationship.
I hope this has helped you,