Class features don't normally continue to scale if you multiclass. So you might want to add, say, a cosmic ability (I figure that might be about right) to allow it (for one class feature). HOWEVER. - Not all class features are created equal, and this may cause the rules to spontaneously explode:
Scaling powers are good because, as long as whatever you compare them to scales at the same rate, they will always do the same thing. But, in allowing 2 abilities that normally should not scale together to do so, you may create a situation where both of those abilities, in tandem, work together to cause
Bad Things to happen.
Example: Both Sneak Attack and Smite Evil add damage to an attack and scale ~ Linearly. Meaning they mean about the same thing at level 2 as they do 20. Adding them together, however, you may find that +1d6+2 damage / two levels is a ridiculous sum of damage ON TOP OF normal damage. Or Monk/Swashbuckler AC bonuses. Or pretty much everything that could potentially be used WITH another class feature potentially useful at the same time.
I would recommend, if you wish to offer this to players (or balance monsters with it), that you price such a capability as Cosmic, and require the person taking the power to Specify a specific class feature. I think anything lower than a cosmic ability would be too cheap a cost to a character and would lead to a 'pile-it-on' scenario. Also it might become too common and dilute class worth. "You are a Cleric? Well Bob over here is a Fighter with all of a Cleric's powers. He's like you, except much better."
Plus, in several
WotC supplements for 3.5 (PHB 2?), they present feats for doing just this but with specific and (hopefully) carefully chosen class features.
Linear Mode
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