I dunno. I'll probably include skills and weapon/armor proficiencies in the downtime rules. Easy enough to explain the weapon/armor ones as going to "boot camp". I may make it so that you have to get armor proficiencies in tiers. Light first, then medium, then heavy. So if you're a wizard and want to wear plate mail, go take basically 3 years off and learn it. Time doesn't stand still for you, though.
I'll probably do the same thing for multiclassing as well. Going to bed one night a fighter and waking up the next day with magic user skills never set well with me.
I plan on getting around the extended downtime bogging down my campaign with the use of henchmen. If Ivor the Terrible, Barbarian of the North wants to take time off to learn basketweaving (or whatever), his traveling companions can be joined by Ivor's henchman, Doman the cleric of Ooh.
Back in the day, we all played with multiple characters.