I love Mystara for D&D, but it very much is a setting built for "I have the gazetteers forenjoy these 5 things out of the total output, so we're running those areas, the rest is interstitial space between them." Even moreso than the other D&D settings, it is not a well-oiled machine that both holds together but also falls apart if a piece is removed. If you personally don't like Glantri, they can easily just be 'oh, there's a nation of wizards over there, they, uh, keep to themselves. I guess fear of them keeps everyone else from committing to full-scale war on their other borders (something something).'