I didn't expect to see it in Warframe proper, but something like an open-world Warframe is something that I've been seeking for a while - I've been on a bit of an open-world bender recently, especially with LoZ: Breath of the Wild, and combining that kind of explorative map with Warframe's incredible core gameplay loop was something that I thought would be amazing to see in action. Didn't think DE would do it themselves, though :D
Supposedly, the plan is to see how PoE goes, then give each planet an open-world zone if people end up enjoying it. This is something I'm wholly in favor of.