Losing contestants from years ago already are allowed back. I think the length of questions does need to be addressed - it used to be the case that they were strictly timed to give everyone a fair chance, but I think they've become rather lax on that recently. Incidentally, I recently watched the 1990 final on video and it actually surprised me how similar the standard of general knowledge questions was to now. I'd imagined they'd got much easier over the years, but on that evidence I no longer think they have.