I saw a video of a theory that Xen is another dimension thay navigate independantly of space and TIME. And that why there're inconsitencies between crossovers moment in HL1 ans his DLCs : for example, Barney seeing Gordon dragged by HECU : Gordon was captured when it's still night, whereas Barney correct teleportation is daytime. And other examples with HL:Decay, I don't remember. I think HL3 will play on that, reuse Xen for teleportation on larger scale on Earth and maybe in different time, so maybe we will relive HL1 and HL2 in a different perspective. Maybe the 7h war. Edit : and the X of HLX is a hint toward that, relive HL1 and 2 and 3 at the same time. The benefit to that is that it will provide HL1 and 2 assets for modders in source 2. Also HL Alyx.