I've actually always wanted to see the CBJ-MS in ArmA.
I'd like it because:
1. It looks great ()
2. It's swedish (patriotism engage)
3. It's really powerful with its 6.5x25mm cartridge, it punches cleanly through the armour of an MT-LB:
There are loads of awesome swedish weapons and vehicles (JAS 39 Gripen *cough*) but I've only seen our rocket launchers in games before, and I think it would really add something to the series, and not just for me, for I believe that quite many people would like something swedish in ArmA.
One more thing about the ammunition: You can easily convert almost any weapon that uses 9x39mm para to use 6.5x25mm, thereby making an Mp5 an assault rifle, more or less.
Links and info:
Wikipedia
world.guns.ru
Official page of the weapon
Official page of the ammunition
Ballistic gel testing of the ammunition
Armour penetration testing of the ammunition
Information on the weapon
Field stripping the weapon