In this industry never say never. Though it would be a pretty big task to get those shows back on the air. Right now I'm just hoping the SGA movie actually gets made and released - which is looking slim more and more as days pass.
SGU was made once the crreators decided that they wanted to Take the SG franchise in a different direction. Some consider it to be a soap opera in space, and while I can see that might be the easiest explanation for it, and I feel it does hover that line with not much space. I understand why they went in a different direction, I just don't buy their reason for canceling SGA regardless if it was financial: cast budget, overall buget, etc. or any other reason. I watched the first 9-10 episodes of SGU and got bored. The only character that was remotely likable was Dr. Rush. Though his constant outbursts and hot-headedness was quickly tiring and made me want to change the channel. Though Robert Carlyle is a great actor, and am amazed what he did with his role, and I think he's probably the only reason people are still watching.
I think SGA writers were running out of new and exciting ideas. Sure you never completely run out of ideas for a TV show, but the longer it runs you have to really push to keep it fresh. While the premise of the SGA series was pretty basic: Team finds something, Team member(s) gets trapped or kidnapped, etc. Team must figure out way to rescue "team member(s)", team member(s) rescued (usually). While it was that format that was basic, there's still a lot they could do.
Warning Spoilers are present.
I would have liked to have seen them do something with the Genii and/or Todd the Wraith, played by Christopher Heyerdahl who if you did not know also played Halling of the Athosians in the first few episodes. He also played in one SG-1 episode as the character Pallan.
The Genii were pretty good semi-bad/semi-good guys and that storyline could be it's own show!
They could have even introduced a brand new more advanced race. S5 Ep 10 & 11 were my one of my favorites because of the you know who those awesome armor suits!! :) Maybe a more human race but with technology like the you know who :)
I think a new SG series with a new team but old format would be a great way to continue old races and also introduce new ones. I have one idea of make it more in the future... not too far, but you got Atlantis now back in space, floating - deserted and a new team from SGC enter atlantis. They could even return to the Pegasus galaxy and meet some of the friends.
I think the only way to bring Atlantis back is add new storylines and new team, with of course guest apperances by our Favorite SG-1 and SGA characters. Though I think McKay probably should be part of the "new team" after all he is the reason for everything! :D