Volition's multiple unnamed/unbranded engines for the Saints Row franchise.
The RenderWare fork used for Crackdown 1. The unknown engine for Crackdown 2 (presumed to be an iteration of the Crackdown 1 engine). (Crackdown 3 used Unreal Engine 4. Useful to mention for those elsewhere in this thread concerned Unreal "can't" do it.)
You can argue that BioWare has tried its raw best to push EA DICE's Frostbite engine as much as possible in that direction. DAI and MEA didn't have truly seamless open-world games, but they tried to get the biggest scale of levels that Frostbite seemed capable of and at their best felt "close". (Obviously at their worst they felt nothing at all like GTA/RDR level of detail/size.)
Volition's multiple unnamed/unbranded engines for the Saints Row franchise.
The RenderWare fork used for Crackdown 1. The unknown engine for Crackdown 2 (presumed to be an iteration of the Crackdown 1 engine). (Crackdown 3 used Unreal Engine 4. Useful to mention for those elsewhere in this thread concerned Unreal "can't" do it.)
You can argue that BioWare has tried its raw best to push EA DICE's Frostbite engine as much as possible in that direction. DAI and MEA didn't have truly seamless open-world games, but they tried to get the biggest scale of levels that Frostbite seemed capable of and at their best felt "close". (Obviously at their worst they felt nothing at all like GTA/RDR level of detail/size.)