While reading this thread on RPGSite, I got the idea of running an Ars Magica style game using D&D or a retroclone. For those of you not familiar with Ars Magica, it's all about mages running their own covenant. They research spells, acquire new sources of magic, and often get involved in politics with both local rulers and other magical covenants.
Mages are already pretty powerful in the OSR, but in this type of game, they should likely start at level 5 or maybe even higher. Their bodyguards would be low level fighters and thieves. Clerics may or may not be friendly, based upon the setting. If I ran this, I'd likely limit clerical magic, as well.
Things that are important:
The Library. Seriously, you need to have one and need to use it, or you won't learn any new magic.
Item creation may or may not be included, though I like the idea of it. Especially if you use a skills system, where one mage is a smith and another is a bookbinder.
Sources of magic. It might be a hidden glen in the woods, or it might be the finger bone of a powerful saint. Either way, the source provides the mages with extra power. Maybe their spells do more damage or last longer. Either way, other covenants will be looking to acquire the source from you.
Lots of politics. The local ruler or church may or may not be aware of who you are. The other covenants in the area definitely do. You may have to deal with them without blowing your cover.
You've heard rumors that the Ring of Aladar, a powerful artifact used by an ancient mage, has resurfaced in a nearby ruin. You need to find it and get it. Other covenants and maybe a thieves guild are also looking for the item.
A grimoire from the old empire has shown up in the local bazaar. A local priest wants it burned.
The Duke's family has been assaulted by magic. He knows of you, and asks your aid. Failure to help will likely get you killed, but finding the evil spellcaster could be very dangerous.