If you have a fetish for pistol grip stocks, check out the Speedfeed III and Speedfeed IV stocks.
Also, if you don't have ghost ring sights, check them out. Ghost rings are not for everyone, but I love the one's on my 590.
Another option that I've seen on other Mossbergs is that heat sheild thing. I'll admit that it looks pretty cool, but I don't see a purpose to have a heat sheild on a HD 'gun (unless you plan on defending against 50 attackers).
I've looked at the Responder dedicated flashlight forearm from SureFire and I'm not impressed. From what I understand, you may have to chop off the bayonet lug in order for the light to work. I think SureFire now has a version that dosen't require lug amputation. I currenly have a project where I will use the existing bayonet lug to mount accessory rails.
Didn't you know that slings are usually only for rifles? What you probably want is a carry strap. My 590 has the Unky Mikes shotgun carry strap with the handy shotshell loops. The strap also has a nice no-slip materal on the backside of the padded section.
Have fun tricking out your 590.