With some still getting over the demise of SG1, its successor, Atlantis, is to finish after 5 seasons. Atlantis, like SG1, will continue in direct-to-DVD movie format.
I've grown to like Atlantis, so this is a real shame. However, Stargate is not going the way of the Trek franchise, at least not yet, it continues with Stargate: Universe.
The premise of SG:Universe is that the Ancients (who created the Stargates, devices which can provide instant long distance travel and which are the fundamental conceit of the show) decided to expand the Stargate Network.
They had already planted Stargates throughout the Pegasus galaxy (from Atlantis), through the Milky Way (from a base on Earth), and then decided to spread the network through the universe. They did this by sending a ship out to plant the gates, which would then be followed up later on. For the ancients, later on never happened - and the ship continues to this day. The premise of SG:Universe is that Earth discovers how to send a mission to the ship - not realising that there was no way to 'dial home'. The mission is therefore stranded, on a ship with a pre-determined mission (i.e. the ship leaves you behind, tough!)
In the series, we discovered the Milky Way network first, then found out how to connect back to the Atlantis gate. This was to be a one-way trip, but two way travel was worked out (even if it wasn't instantaneous.
Universe could promise to do what Atlantis promised originally, that is to have a completely 'cut off' scenario, separate from Stargate Command.
However, there is an interesting possibility given the established idea that a Stargate is 'one way'. In other words, it has been established that once A opens a stargate to B, then stuff can only go from A to B, not from B to A - but radio is 2 way. This could be 'explained' as the ship isn't a fixed point of origin combined with some bit of technobabble about a ninth chevron on the Earth Gate.
This means that there is the potential for reinforcements and supplies being sent from Earth, and intelligence flowing back from the ship - but anyone joining the mission knows that it is a one way deal - this could give rise to some interesting scenarios. For example, what if the 'Universe' commander is to be replaced by Earth, but the people on the ship refuse to accept the replacement? What if the (presumably international) ship crew knew that earth was having international problems, and their radio link was severed for some time. They might presume a nuclear war on Earth. Would the tensions spill out onto the ship?
In the Stargate Universe (as opposed to Stargate:Universe), we have seen existing characters in one series moving on to another. David Hewlett's character of Rodney McKay went from a minor SG1 character to a major part in Atlantis, for example. I'd hate to see the back of Rodney McKay - and would welcome him on SG:Universe - but, to continue this theme, I've been thinking about which relatively minor existing character I'd like to see move to Universe. This makes sense from a story point of view - a new military expedition would seek experience, but there would also have to be thought to the existing teams.
Certainly, to earn his stripes, Zelenka would have to save the day in Atlantis at least once and show Rodney to be wrong in the process (have we have any purely Zelenka ideas which weren't immediately spotted to be Bad by Rodney?)
It'll be a shame to see Atlantis go. Universe, like Atlantis before it, will be a risk - I hope it'll be a risk that pays off - I've got used to seeing Stargate on my screen.