Does a metamagic feat that does this already exist?

[Yqatuba]

Is there a metamagic feat that increases the HD limit of a spell if it has one (such as sleep/deep slumber)? If not, how much should it increase by? 50 percent like Empower Spell?

[avr]

No such metamagic feat exists, but a couple of class abilities are kind of similar. The mesmerist's mental potency extends HD limits and increases total HD affected by 1 per 5 levels, the heavens oracle's awesome display reduces the effective HD of the target by their Cha mod for the purposes of HD limits.

I'd be wary of creating something which might stack with either, especially awesome display.

[Meirril]

Generally speaking, it would be better to create a higher level version of a spell that you want to add HD to than creating a way to add HD to any spell that is limited by it.

Spells with HD limits have them for a good reason. Creating a higher level version lets you judge just how much improvement has happened. Also the DC will increase. Also there are rules for researching spells, none for researching new feats.