Epic has had their Unreal engine working flawlessly on mobiles for a long time now. Since then, mobiles have only gotten faster and it will only continue. Epic is good at creating very efficient code capable of squeezing maximum performance from hardware with minimal waste. A quick glance at low quality Second Life without the lighting, shadars, and extra features and I see absolutely zero reason that couldn't be brought over to a mobile platform today. Realistically (and it's already been mentioned), the only problems would be streamlining the UI and power consumption. To address those problems, you need some sharp usability experts. As for power consumption, this has already been discussed at length.