Today when creating a "Custom field" we can select "For all projects". This is good but when I want to activate only for a subset of existing projects, I have to go to the Settings of each project and activate it. To give a scale I often have to activate for 30 project for a total of 40 existing ...
Moreover, when I select "For all projects", it can not be unselected afterwards from Project page (which would have done here 10 updates only instead of 40...)
I often finish operating direct sql insertions into db which is not my favourite way of doing the administration of my platform.

It would be nice to have in the Custom field form the list of the projects appearing (eg like Watchers do in Issue Creation form) and be able to select these.
Furthermore the "for all projects" should not be a parameter of a custom field anymore, and instead we would have "select/unselect all" over the list at the bottom. This would avoid conflicts/uncoherence in Redmine configuration and also any Custom field could be deactivated in the settings of a project all the time and for all the Custom fields. More flexible.