Jeff Rients has the second of a series on a campaign at the beginning of history.
I had a random thought reading it. One of the things people claim to like about B/X is the limited spell selection. Magic-users get one spell per daily slot and have to specialize. There is a great deal of argument about spell research allowing you to exceed this count or not.
A dawn age campaign can get the same effect without limiting spells known by a different method: limited spells lists. At the beginning of time no one will have created a lot of spells. I would consider limiting the spell lists, perhaps even beyond those in B/X. While lots of 1st level spells might be known I’d trim the lists as spell level went up.
Given OD&D only went to level 7 (or was it level 5, I don’t have my books here) I’d work from highest level down. There would be 1 7th level spell known, 1 6th level, 2 5th level, 2 4th level, 4 3rd level, 4 2nd level, and a mere 8 1st level spells. All others would have to be researched or learned from their creator. You can choose different numbers but you get the idea.
This would also allow a variant of 2nd edition type specialist magic users from the Unearthed Arcania netbook. Instead of specializing by college, specialize by tradition. In the netbook a tradition is based on spell names that contain a magic user’s name. Joining a tradition might give access to twice the spells or even the first 8th and 9th level spells. It could also provide an endgame option. Allowed a name level magic-user to create a tradition if he has researched his own spells of each level available and specialize in his own tradition.
You could get more complicated such as spells from a previous tradition founded by his master and so on. The idea is to have the players’ magic users inventing the magic of the ages that follow instead of digging up that of ages that preceded.