All of these things relate to your points: you want them to contribute to the creative process, which ultimately results in a better product, and makes them feel like they’ve made a positive contribution. Everyone wins: you get hired again because you deliver a good product while making them feel good.
The thing is, you can’t tell if an idea has legs until you share it. And even after going through your own filter and that of your colleagues and other disciplines, the only thing that’s actually important is how the product is received by the audience.
So holding it all close to your chest is just a really good way of reducing the likelihood of success. And because of this, wherever I can, I try to mock up a cheap prototype of a product first, something that expresses my ideas in a tangible way that others can understand, just to make sure that what I’m working on is actually worth the time.
For example, replacing a table view that gets limited engagement with a feed with some workflow actions.
Half baked at this point might be some static screenshots from figma.
It can go wrong in a few ways.
Probably the worst way is where the customer takes a half baked demo or idea as a preview and a commitment, and then demands that you deliver it, or complains when you don’t; not all half baked ideas can be turned into a product.
Another potential pitfall is that the idea may be seen to represent a change to something important to them, and they get scared. Inducing needless fear is best avoided!
They might just tell you that it’s a stupid idea. Which at best is just embarrassing.
There are probably many other negative scenarios, but those come to mind from experience.
So while on the whole I think you really, really should let customers be part of the ideation process - they are the users and beneficiaries of the work, after all - you have to chose who you include carefully, because the relationship with your customers is fundamentally different to the one you have with your coworkers, and it can go south in really unexpected ways.