The worst thing you'll get from a bad suggestion is a temporary ban from that particular forum. That pretty much only happens when you're blatantly ignoring rules you're well aware of. You certainly won't suffer penalties to your character, or your overall SWC access.
The issue of the XP system is a tricky one, as generally any changes relate to changing the entire experience/skills system, which won't happen and is restricted for that reason.
If you want to make a suggestion about XP gain within the existing system, that is not against the rules.
If you make a suggestion like "XP from space travel should only be used to level up space skills", that's a major change, and not going to happen.
If you're ever unsure whether your suggestion is allowed or not, you can always drop the forum moderator a DM with a brief outline, and they should be able to tell you whether it violates the rules of that forum.
____________"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
Only fools and children dream of heroes.