AROS on any x86 board..
I tried AROS and at the time I figured since MOS was a flash in the pan, AROS was the next best thing once UAE integration was built in (whatever they called it). Now that MOS2 is here, its very polished. Yea it has bugs and yea, they'll be fixed.
To combine OS4 coders (I think you mean the brothers from Hyperion?) + MOS, to put it mildly, is a absurd idea. OS4, its features, software, design has nothing new or good to offer MOS. MOS Devel is a rag-tag group of hardcore coders. I may call them elitists because they dont speak up in public w/news regarding MOS but I respect them highly for what they've designed. They do not need any design help or ideas for MOS.
OS4 had its chance and AINC got greedy and is burying it in the deep waters of the legal ocean, unfortunately.
Anyway, come on over and check out MOS if you havent, it freakin rocks. It will make you feel like the first day you bought your Amiga.

As for MOS having Amiga IP in it.. if they knew for a fact they would win the case or had a chance, they would of buried MOS.