Generally when I think of multi classing, I think of giving up something in one class to gain something in another. This is what the 4e system currently lacks, you are basically just spending feats to get new powers, and you never get enough blend to fit many archetypes.
So here's my idea. Take the core class, and swap out the class abilities with that of another class.
Here's how you do it:
1) Start with a primary class. At 1st level, you can choose to multi class.
2) You keep the saves, weapon/armor proficiencies, hit points, and powers of the original class.
3) Choose a new class. You gain the skills and class abilities of the new class.
4) You must choose which class you count as for purposes of feat prereqs and paragon paths. Once chosen, you cannot change this.
5) You can now take the multiclass feats to pick up powers in your new class.
Special Notes:
1) If your primary class can use implements, you do not lose them.
2) If your primary class has a build option (for example the Rogue's Artful Dodger) you still count as that build option for the purpose of powers and feats you take. You simply don't gain the class benefit of that option.
3) If you pick up the warlock's class abilities, you do not gain a new at-will from the pact you choose.
4) In this version, the wizard's spellbook could apply to any power type, not just arcane spells. Further, arcane implement mastery applies to any power that uses an implement, not just wizard spells.
Let's do some examples:
1) A fighter decides he wants to be a bit more tactical and leaderish. He decides to multiclass into warlord, becoming a "Fighter Lord". He keeps his hitpoints, armor, and weapon type, and his +2 to fort saves. He gives up his combat challenge and superiority and most importantly, his +1 to attack rolls with his weapon. In return he gains the warlord's +2 init aura, inspiring word, warlord skills, and chooses to become a tactical warlord, gaining the bonus to attack rolls for allies who spend action points. He still uses fighter powers. For feats and paragon paths, he decides to become a warlord, and picks up the tactical warlord feat that allows him to add his int to damage from attacks that use action points.
The character has lost the main defender role of the fighter, and especially the +1 to attack rolls. In exchange, he can now heal and aid his allies on the battlefield somewhat, though he still mainly uses the hard hitting fighter powers as his main stay.
2) An artful dodger rogue decides to multi class into warlock for a different kind of striker. He gives up the artful dodger's charisma bonus to AC against OA (however he is still considered an artful dodger for purposes of feats and powers). He also loses the rogue's sneak attack, weapon talent, and first strike abilities along with the rogue's good skills. In return he gains the warlock's curse, shadow walk, prime shot, warlock skills, and the pact ability. He does NOT gain the pact ability's at will though. He decides to stay with rogue for the purpose of feats and paragon paths.
We now have a new hybrid of striker, one that combines the stealth of rogue powers with the cursing of the warlock.
3) A cleric decides to multiclass into wizard. The cleric gives up his channel divinity, healing lore, healing word, ritual casting ability. He gains the wizard's cantrips, ritual casting, arcane implement mastery, and spell books. The cleric now uses a prayer book, and can decide dailies and utility powers as a wizard can. He also takes the orb as his mastery, letting him use his impressive wisdom. Also, the cleric can use holy symbols or the new implements of the wizard.
We now have a cleric that has given up his hardcore healing for a wider variety of rituals and magical effects. He can still heal somewhat with his cleric powers, but not as well...though he also gains impressive use of the orb power with the cleric's already high wisdom.
4) A wizard multiclasses into fighter. He gives up the spellbook, cantrips, arcane implement and ritual casting. He gains the fighter's ability to mark with his attacks, and the +1 to attack with weapons. The wizard can still use implements, he just doesn't get any extra bonus for his arcane implement mastery anymore.
The wizard has become a battlemage. He has given up a lot of the wizard's magical power, but gained the impressive ability to mark large numbers of targets at range.
Final Thought: A fighter with a spell book?
Obviously this idea begs the question, how do you describe a fighter having a spell book? The answer is...anyway you like. Perhaps the figher is a martial student, one who learns a variety of secret techniques through ancient manuals and personal accounts of sword masters. While he doesn't have the natural ability of many fighters (no +1 to attacks for example) he gains a wider breath of attacks (ability to choose dailies and utilities each day).
So here's my idea. Take the core class, and swap out the class abilities with that of another class.
Here's how you do it:
1) Start with a primary class. At 1st level, you can choose to multi class.
2) You keep the saves, weapon/armor proficiencies, hit points, and powers of the original class.
3) Choose a new class. You gain the skills and class abilities of the new class.
4) You must choose which class you count as for purposes of feat prereqs and paragon paths. Once chosen, you cannot change this.
5) You can now take the multiclass feats to pick up powers in your new class.
Special Notes:
1) If your primary class can use implements, you do not lose them.
2) If your primary class has a build option (for example the Rogue's Artful Dodger) you still count as that build option for the purpose of powers and feats you take. You simply don't gain the class benefit of that option.
3) If you pick up the warlock's class abilities, you do not gain a new at-will from the pact you choose.
4) In this version, the wizard's spellbook could apply to any power type, not just arcane spells. Further, arcane implement mastery applies to any power that uses an implement, not just wizard spells.
Let's do some examples:
1) A fighter decides he wants to be a bit more tactical and leaderish. He decides to multiclass into warlord, becoming a "Fighter Lord". He keeps his hitpoints, armor, and weapon type, and his +2 to fort saves. He gives up his combat challenge and superiority and most importantly, his +1 to attack rolls with his weapon. In return he gains the warlord's +2 init aura, inspiring word, warlord skills, and chooses to become a tactical warlord, gaining the bonus to attack rolls for allies who spend action points. He still uses fighter powers. For feats and paragon paths, he decides to become a warlord, and picks up the tactical warlord feat that allows him to add his int to damage from attacks that use action points.
The character has lost the main defender role of the fighter, and especially the +1 to attack rolls. In exchange, he can now heal and aid his allies on the battlefield somewhat, though he still mainly uses the hard hitting fighter powers as his main stay.
2) An artful dodger rogue decides to multi class into warlock for a different kind of striker. He gives up the artful dodger's charisma bonus to AC against OA (however he is still considered an artful dodger for purposes of feats and powers). He also loses the rogue's sneak attack, weapon talent, and first strike abilities along with the rogue's good skills. In return he gains the warlock's curse, shadow walk, prime shot, warlock skills, and the pact ability. He does NOT gain the pact ability's at will though. He decides to stay with rogue for the purpose of feats and paragon paths.
We now have a new hybrid of striker, one that combines the stealth of rogue powers with the cursing of the warlock.
3) A cleric decides to multiclass into wizard. The cleric gives up his channel divinity, healing lore, healing word, ritual casting ability. He gains the wizard's cantrips, ritual casting, arcane implement mastery, and spell books. The cleric now uses a prayer book, and can decide dailies and utility powers as a wizard can. He also takes the orb as his mastery, letting him use his impressive wisdom. Also, the cleric can use holy symbols or the new implements of the wizard.
We now have a cleric that has given up his hardcore healing for a wider variety of rituals and magical effects. He can still heal somewhat with his cleric powers, but not as well...though he also gains impressive use of the orb power with the cleric's already high wisdom.
4) A wizard multiclasses into fighter. He gives up the spellbook, cantrips, arcane implement and ritual casting. He gains the fighter's ability to mark with his attacks, and the +1 to attack with weapons. The wizard can still use implements, he just doesn't get any extra bonus for his arcane implement mastery anymore.
The wizard has become a battlemage. He has given up a lot of the wizard's magical power, but gained the impressive ability to mark large numbers of targets at range.
Final Thought: A fighter with a spell book?
Obviously this idea begs the question, how do you describe a fighter having a spell book? The answer is...anyway you like. Perhaps the figher is a martial student, one who learns a variety of secret techniques through ancient manuals and personal accounts of sword masters. While he doesn't have the natural ability of many fighters (no +1 to attacks for example) he gains a wider breath of attacks (ability to choose dailies and utilities each day).
so there's some ulterior motive 
. Here we go!
, but other than skills, this is strictly superior levels 1-10.
