Currently in the Idea Form there is no way to have a field where we have pre-defined values and new ones could be added. (predefined tags with new values would work as well, that's not available either.) Would really appreciate to have one such field.
Thank you for your idea!
The intent of not allowing ideas portal users to add more options to a choice list is to give you the power to control the options. However, your use case does make sense.
We will continue to monitor this idea for community feedback and overall prioritization.