2) Advance faster in the new class because 1st level is cheap. Don't do this. Period. It will break 5E. If the PC makes it to 5th level as a fighter, then switches to rogue, his next level is 6th. There should not be a way to avoid this. Even if you use #1, above, the character will eventually get to 6th level rogue and have no penalty for using fighter abilities, which makes him an 11th level character. He made it there with only 15% of the XP as the pure wizard took. I don't care how you slice it, that's broken.
I take umbrage on this specifically. The Fighter 5/Rogue 6 is not an 11th level character. He has 6th level HD (5 Fighter, 1 rogue), has 6th level proficiency bonus and lots of options but still a single action to use them with - I call this "Broader Options, Shallower Specifics".
I'm in the process of writing up Dual Class / MultiClass taken from 1st Ed for 5th. I was hoping the discussion here would provide some direction on where I'm unclear on what to do - but instead I just see grognards boohooing a system they obviously won't adopt. Instead of being negative, just don't post. It'll help keep the discussion clean.
I haven't crunched the numbers yet, but what I'm looking at is doing this:
For Dual Classing, limiting it to two classes - any combination. Level as far in the first as you want, gain all abilities/proficiencies, HD, etc. Second class, XP resets to 0, you can't use any of the first class abilities except HPs, Saving Throws and HD for resting. Second class, once it surpasses the first, grants all abilities, proficiencies and saving throws - if you have a duplicated save, you gain advantage on saves for that attribute. If you have a duplicated skill, you can pick another skill (pretty sure that won't happen, as the skill lists are more extensive than the number you get - but just want to cover the bases in case another class comes along that necessitates it).
Spell casting: You gain two sets of distinct spell slot tables. A Cleric 5/Sorcerer 6 gets slots for each, and can not use cleric slots to cast sorcerer spells, or use sorcerer spell points to regenerate cleric slots.
For Multiclassing - I've created 4 class designations: Warrior (Barbarian, Fighter, Ranger); Priest (Cleric, Druid, Paladin); Mage (Sorcerer, Warlock, Wizard) and Knave (Bard, Monk, Rogue). A multiclass character can only pick one class from a designation. You can be Duo (Barbarian/Bard) Tri (Ranger/Druid/Warlock) or Quad (Fighter/Cleric/Wizard/Rogue) for example.
As I noted, I haven't crunched numbers yet, but the idea is that a duo class should reach 17/17 when a single class hits 20. A tri should hit 14/14/14 and a quad hit 11/11/11/11 - so I'll run numbers and create levels of advancement compared to a single classed character.
For abilities, the MC gains access to all their classes class abilities. They gain skill proficiencies equal to the most given for a specific class. A tri gains 1 additional, and a quad gains 2 additional skills, from the whole list of class options. Saving throws: Pick any two from any class available. A quad class gets a third choice. HPs are averaged - either rolled or the base provided in the PHB.
Spellcasting: Only have a single table, as shown for multiclassing in the PHB, however, any half or partial casters add to the table.
A 6th level Paladin (half)/Rogue (Arcane Trickster (partial)/Wizard (full)/Cleric (full) would be 6 (wizard/Cleric)+3 (Paladin)+2 (Rogue) = 11th spell slots to power their 3rd level and lower spells. Note, this character is on par with a 13th or so character, so their slots are still less than a single class caster - again, Broad Options, Shallow Specifics.
I haven't run this by my players yet - wanted to get the specifics down first - and was thrown for a loop trying to do experience tables by class (thrown out for simplicity sake). But at least the idea is intriguing to them.
