Originally on gleemax, but I think this board will appreciate it more.
What follows is my impression of a multiclass system which works and isn't greatly underpowered. If you're going to claim its broken, please prove it rather than just asserting it. Of course its more powerful than the current system, the current system is made of fail, so comparisons to the current system will be ignored.
I am of course also open to any suggested tweaks others may think of.
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Feats
Multiclass (X)
Prerequisite: 13 in an attribute used for at least one attack power to-hit roll by the chosen class
Effect: Choose a class other than your own. You are now multiclassed into that class.
You gain skill training in one skill normally available to the chosen class.
You must swap one of your at-will powers with one of the chosen class's at-will powers.
You may use implements appropriate to the new class for powers which require them, if any.
Special: You may only choose this feat once.
Novice Power
Prerequisite: Multiclass feat, 4th level
Effect: Replace an encounter power from your primary class with an encounter power from your secondary class of the same power level or lower.
Additionally, choose a skill available to your secondary class. You gain a +2 feat bonus to skill checks with that skill. You need not be trained in this skill.
Acolyte Power
Prerequisite: Multiclass feat, 6th level
Effect: Replace an utility power from your primary class with an utility power from your secondary class of the same power level or lower.
Additionally, choose a skill from your secondary class that you are not trained in. You are now trained in that skill.
Adept Power
Prerequisite: Multiclass feat, 8th level
Effect: Replace a daily power from your primary class with a daily power from your secondary class of the same power level or lower.
Additionally, as a daily power useable as a free action, you may replace one use of an attribute in a power from your secondary class with a key attribute of your primary class (your choice for which attribute). For example, you might choose to use a different attribute for the attack roll, or you might use a different attribute for a secondary effect. However, only one use of an attribute is changed.
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Paragon Paths
If you are multiclassed into a class, instead of pursuing one of your own Paragon Paths you may pursue a paragon path of that class. You count as meeting all class and class feature pre-requisites, even if you don't have them.
Alternately, you may choose your multiclass as your paragon path if you have the novice, acolyte, and adept power feats. You are now fully multiclassed into the new class, and equally count as your original class and your second class.
Paragon Full Multiclass
At 11th level you gain any one encounter power from your second class of level lower than your own.
At 12th level you gain any one utility power from your second class of level lower than your own.
At 20th level you gain any one daily power from your second class of level lower than your own.
All such powers are class powers, and when you subsequently gain a level which allows you to replace a class power with a higher level one, you may choose to replace these. Whenever you replace a power you must choose a new one from the same class.
Additionally, paragon full multiclassing grants you access to class features of your second class as follows:
Cleric
11th: Healing Word, Healer's Lore
16th: Channel Divinity
Fighter
11th: Combat Challenge, Fighter Weapon Talent
16th: Combat Superiority
Paladin
11th: Divine Challenge, Lay on Hands
16th: Channel Divinity
Ranger
11th: Fighting Style
16th: Hunter's Quarry, Prime Shot
Rogue
11th: First Strike, Rogue Weapon Talent, Sneak Attack
16th: Rogue Tactics
Warlock
11th: Eldritch Pact, Warlock's Curse
16th: Prime Shot, Shadow Walk
Warlord
11th: Commanding Presence
16th: Combat Leader, Inspiring Word
Wizard
11th: Spellbook*, Cantrips
16th: Implement Mastery
All class features gained function exactly like they would if the class were your primary class.
*Spellbook becomes retroactive for any other wizard utility or daily power 'slots' you have.