In Astonishing Swordsmen & Sorcerers of Hyperborea (and other old school games of its ilk) a Magician (nee Magic-User) gains one spell automatically when they increase in level. They may learn other other spells, of course, but those must be obtained from plundered spellbooks and scrolls. I propose a slightly alternative rule that doesn’t provide a Magician with automatic spell acquirement, but does increase the odds of gaining more than one spell upon leveling up.
The Magician chooses 3 spells that they would like to learn upon increasing in level. These are spells that the Magician is considered to be researching throughout their downtime. The 1st spell has the Magician’s base chance to learn a new spell (50% for Magicians of Intelligence 9-12, 65% at 13-14, and so on). Each subsequent spell has a progressively reduced chance of being learned by 10% up to the 5th spell which is reduced by the base chance by 40%.
For example, a 1st level Magician with a 13 Intelligence (base chance 65%) would choose 5 spells to study as they adventured. Let’s say they knew Magic Missile, Grease, and Hold Portal already. So, they wanted to learn Identify, Charm Person, and Protection from Evil. They could sort them as follows:
|Protection from Evil||45%|
When the Magician reaches 2nd level, she rolls for a chance to learn each spell. She rolls a 69, 7, and 42 and therefore learns Charm Person and Protection from Evil but not Identify.
Of course, you could expand upon this idea by giving the Magician the option to continue researching spells they failed to learn. They may continue to research a spell and thus gain a +10% bonus from the previous chance they had. In the example above, the Magician might add two new spells to learn at 3rd level, Flaming Sphere and Invisibility, but keep Identify in the running at 75% instead of 65%.