While I don't agree at all that CoE wrapped things up (personally for me the ending felt hollow and rushed - like there should be another episode afterwards. Or at least another scene. Only, there wasn't) However, I'm not sure I'd want to see another series.
Mainly because I don't think the series would work if it was just Gwen and some new people. She's not a strong enough character without Jack/Ianto to balance her out, and any attempt to "recreate" a Jack or Ianto figure would have people (including me probably) complaining. And if they [i]did[i] bring Jack back, any attempt to give him a love interest would be hated by Janto fans.
I really didn't like CoE, but I'd be afraid of any further series ruining Torchwood even further. But that's just me, I know plenty of people did like CoE!