Classes and Races.
My feeling here is that in cobbling together a system from other parts, a Frankensystem, an area that deserves a look is the races and classes that should be available in the book. Of course the DM can rule stuff out, but it should be a choice not a limitation.
The races that seem to have the most resilience across almost all of D&D and being most associated with more of the old-school games are the following:
Dwarves, Elves, Gnomes, Half-Elves, Half-Orcs, Halflings, and Humans.
No thought has yet been given to level limits. Class restrictions likely would be tossed out in favor of some other balancing mechanic, perhaps experience point costs.
The classes most familiar with old-school gaming and some association with the more modern variants include:
Assassin, Barbarian, Bard, Cleric, Druid, Fighter, Illusionist, Monk, Paladin, Ranger, Thief, Wizard.
Here is where I begin to go against my earlier statement of choice, not restriction.
Anybody can assassinate a target, but someone with specialized training to help complete such tasks as a profession might be a class. However, this could be more of a background professional package that is attached to a class at the time of character creation.
For example, Brother Holden, a cleric, is also secretly a member of the Duke's special services. During play he can justify to the DM why he knows a lot about poison or lock picking, and why, when he takes off his heavy armor he is as sneaky as a thief, and why he knows enough about anatomy to do more damage with a surprise back stab.
This isn't completely thought out yet and is inspired by the way I've heard backgrounds in the 13th Age rpg described.
Likewise, the Cleric class seems more of a militant arm of the clergy and thus Paladin should be a title within the Cleric class instead of a separate class who's claim to fame is stealing a trope from the Druid and calling an animal companion after a certain level.
Druid would most likely be a priest of a nature god. Perhaps they would at character creation sacrifice some of the martial aspects to gain more nature related specialties. Other types of clerics might have a similar ability in order to better fit their deity. Besides, why would a deity of healing b dressed as a walking war machine (I smite him with my great hammer to cast cure light wounds. I do 1d10 damage and cure 1d6 +1 hit points . . . Never call for this medic on the battlefield.)
Rangers are another class that could be turned into more of a background package. A fighter with wilderness scouting knowledge.
Illusionist too could be a specialized background or simply how a wizard sees herself because of her preferred spells.
All of this is nothing more than musing at this point.
Multi-classing is something for another time.