Apparently there's going to be an announcement at SDCC on July 20th, according to stargate.mgm.com. Nothing major on the website, all you can do for now is sign up to get an e-mail alert "when we activate the Stargate", as the site puts it.
There are a few possbilities of what the announcement is:
1) The 1994 Stargate reboot trilogy that was cancelled back in November 2016 is back on track
2) An SG-1/SGA/SGU movie
3) A new season of SG-1/SGA (note: SGU season 3 is happeneing, but in a comic)
4) A new TV series (reboot or sequel)
5) Remaster/Blue-Ray of SG-1/SGA/SGU
6) A Stargate game (either mobile or console/PC)
Minor update:
On July 19th, the official Stargate Youtube channel posted the entire SG-1 pilot episode.