Even if this system is off the table for now, I will finish specifying it in case it gets reworked.
Racial Modifiers To Leveling
While each race does have its drawbacks, the demi-humans are still essentially improvements, mechanically, over humans. It is very easy to create a character around the disadvantages while utilizing the advantages. Hence, I am going to retain the common concept that demi-humans receive some sort of penalty to leveling.
The target numbers to level are modified by race as follows:
Elves, Half-Elves, and Half-Orcs: +1 to target number
Dwarves, Halflings, and Gnomes: +2 to target number
These were mostly divined from each races' benefits. Dwarves, Halflings, and Gnomes all receive substantial bonuses to saves, enhanced detection and/or infravision, and Halflings are practically invisible outdoors. On the other hand, the other races really only get detection bonuses and infravision as their improvements. As I said above, I didn't really take the disadvantages into consideration. Most players will just game around them anyway.
Yes, for now, all the standard races are in and they also have their standard abilities, adjusted of course to fit the rules on skills and whatnot. Ultimately, I couldn't think of a really good reason to fiddle with the choice of races that would offer an overall improvement to the system. Maybe as an option or supplement, but not as core rules.
Multi-class characters receive experience in the same manner, one per session. At the end of each session, the player specifies which class he is trying to level up when rolling to advance. The mechanics work the same for using and retaining experience points as for a single class character.
This mimics how multi-class characters share their total experience points between all classes. Players do receive more flexibility in how to advance and can make characters that focus more heavily on certain classes as opposed to being more or less even in standard AD&D.