I am new to the Sub2000, in fact mine is at Kel-Tec getting fixed so I haven't even shot one yet. That said, I am also into SKS's which have countless mod opportunities and I've tried many of them and usually end up back to the good ole original configuration. Still, it's fun to mod stuff (including chainsaws) so my rules of thumb for modding are:
1. Try it the way it is first, before you mod anything
2. Don't ever trade, sell, give away or throw out the original parts
3. Don't do anything you can't un-do
4. Don't spend a crap load of money on mods, especially for inexpensive guns
5. Don't be surprised when you end up putting it all back the way it was
6. Get advice from people who know (ignore the idiots if possible)
7. Have fun, do what makes you happy
8. Don't worry about what the other guys think
As far as the Sub2000, I got it because it folds and it's lightweight and compact. So for me it would not make sense to add a lot of stuff, especially if it could no longer fold. I am thinking of getting a tactical light/laser combo and maybe tube cover and stock extenion but that would probably be it, no bulky forearms or grips or scopes. That's just me, nothing wrong with that stuff if that's what you want and it does look cool.