I think it would be cool to see older bosses reappear almost as remnants. They're artifacts from a forgotten time and forgotten worlds held together by strings of magic. Re-imagine them as remnants of a bygone era. Their bodies are rotting and their abilities dampened, but they are awake and...
I don't necessarily want the old bosses to reappear in a more traditional boss way, but I think it'd be cool to see creatures like the King Dodongo and Gohma to appear as field bosses within the (assumed) underground areas. It'd be cool to be venturing in some deep tunnels and just see that lone...