In terms of item buying, all they had to do was how XI did it (Yes yes I know bad game shouldn't exist blah blah):

If you're highly ranked in your nation you can buy items from other nations at an inflated price if your nation placed first -- In terms of FFXIV, when you're at the top rank you can buy items from other GC at inflated gil prices due to being non members while members pay in their company seals.

Since it seems the only issue people have GC lock is any item differences, since being in diff GC otherwise doesn't really affect much but a minute seal bonus in battlefields.