I'm not averse to adding a pistol, and the obvious type would be a full auto one. Might be hard to balance it such that it doesn't replace the MAC10 while still being useful.
Full auto pistol eh? The realistic downsides are obviously the high recoil, small clip sizes, and short barrel length (low accuracy) with the upside of getting a lot of rounds downrange fairly quickly (still slower than the mac10). You can always base the balance off of that, and mess with damage numbers a bit.
I however would worry about balancing it with the pistol and deagle before worrying about any of the primaries.
An easier to balance one (in my opinion) would be a three round burst machine pistol which would (realistically) improve accuracy over a fully automatic model, while lowering the total speed you could send rounds downrange.
Just a couple of hasty suggestions.