Magic-users should be able to get a palpable advantage from researching spells and making magic items. In the "real world", we have to make trade offs in terms of time - you can study to become a doctor, or to become a lawyer, for example, but probably not at the same time. If you choose one pursuit, you miss out on another.
|"You'll have to slay the dragon without me, I'm busy."|
Of course, this can be an obstacle in the course of some games, when the group has a limited amount of time to crack a code or stop an invasion. More often, it's no obstacle at all - perhaps some money that must be spent for room and board, and nothing else.
Here's an idea for how you can model this without entirely disrupting the game.
Downtime for Research and Development
Say our resident magic-user, Merlyn, wants to research the invisibility spell. The GM can decide that this will take Merlyn away from adventuring for, say, two game sessions. That means two meetings of the players to play the game. No XP or treasure for Merlyn while he's busy hunched over dusty tomes learning how to become invisible.
In the meantime, the party hosts a special guest hero, an NPC magic-user one level lower than Merlyn controlled by Merlyn's player. This guest wizard does not earn XP, but does get a normal share of the treasure. Each time Merlyn needs to take a break, the guest wizard can step in, always one level lower than Merlyn.
We have now to come up with a schedule for downtime required for various magical operations. Maybe something like:
Researching 1st to 4th level spells - 1 session
Researching 5th to 7th level spells - 2 sessions
Researching 8th to 9th level spells - 3 sessions
Scribing up to three scrolls or brewing up to 5 potions - 1 session
Making most magical items, including armor - 2 sessions
Making magic weapons - 3 sessions
You can use whatever schedule you think is correct.
Other classes that need to train might use a similar schedule. You could allow a fighter or monk, for example, to sit out for a couple sessions so they can learn some new special maneuver.
Downtime for Organizations
This brings up another time commitment - organizations. Clerics are supposed, in some campaigns, to belong to large temple organizations from which they should draw some advantages. The temple should provide some healing, maybe needed equipment or information, etc. To keep this from being an extra ability of clerics that other characters do not enjoy, it can be balanced by the cleric having to take time off from adventuring to serve the temple in other matters. Depending on how useful the organization is, a PC might have to take one of every ten sessions off or one of every six sessions off or whatever off to meet their obligations. The PC gets a benefit, and pays for it by missing a session now and again.
Downtime for Rest and Recuperation
The same mechanic can also be used to model recuperation time, say from a nasty disease or if you are using old AD&D healing rules from damage sustained in combat. The PC misses a session to heal up while a guest steps in to substitute for them.
The fringe benefit from using this mechanic is that you develop ready NPC characters who can step in to become PCs when an existing PC dies. If Pauline the Wizardess has subbed for Merlyn several times, she can become the party's new magic-user when Merlyn is eaten by a dragon.