maybe there might be a conflict due to GPL not "allowing any future extension/restrictions" but this does provide future restrictions
but then you code doesn't "become GPL" and GPL also has a compatibility clause and doesn't require other code to be GPL licensed, just to comply with certain constraints so it should be a non issue
so I guess it should be fine
(also funny side fact, if you take the GPL license then add a clause/restriction it's still valid as the the "no future restrictions" clause if applying to the GPL _with your changes_ because that is the license you have, only if you make a license which basically says "this code is GPL licensed (link to GPL) and following restrictions comply" instead of "this is a GPL with modifications" license does it matter))
but then you code doesn't "become GPL" and GPL also has a compatibility clause and doesn't require other code to be GPL licensed, just to comply with certain constraints so it should be a non issue
so I guess it should be fine
(also funny side fact, if you take the GPL license then add a clause/restriction it's still valid as the the "no future restrictions" clause if applying to the GPL _with your changes_ because that is the license you have, only if you make a license which basically says "this code is GPL licensed (link to GPL) and following restrictions comply" instead of "this is a GPL with modifications" license does it matter))