In advance of the big reveals at open-world game franchise is in development, presumably at regular developer Avalanche Studios.
Just Cause 4 arrived way back in December 2018 to it was selling many of its Western studios to Embracer Group, including the makers of Deus Ex and Tomb Raider.
Just Cause and Rage 2 developer Avalanche Studios was not part of that sale because Square Enix doesn't own them – but the publisher does own Just Cause. Now, in an investor briefing for the fiscal year (thanks, ResetEra), Square Enix confirms that "the Just Cause franchise will remain our IP," and furthermore, it is "developing a new title in the franchise."
We can only assume Avalanche is back working on this new game, and that it is likely Just Cause 5. According to a job listing, Avalanche is working on an unannounced "cutting-edge triple-A open-world game," which may well be the new Just Cause. We'll have to wait and see, however, especially whether it'll be announced at Summer Game Fest.
If you want to know the full Summer Game Fest schedule, you can check it out at that link.