I think it's because Sarah herself so desperately desires it, mentions it every time that she can - and because Andrew in his position can't come out and directly deny anything - that it's become Sarah's wishcraft for her fans to carry over the line for her.
But Zonk, my love. Twenty years ago and about that many pounds ago, I might have nominated myself.
But as it is, I would definitely like to see him remarry to someone with an established career in her own right.
It's my understanding that Prince Bernhardt of Sayn-Wittgenstein-Hohenstein and his wife have recently separated. He brought someone else to the religious wedding of Princess Nathalie. I think she - Katharina Prinzessin zu Sayn-Wittgenstein-Hohenstein - would be a perfect fit! She's very well educated and versed in history and art, having set up two major museum collections; she worked for Sotheby's as a director at one point. She's clearly suited for Royal life, having balanced it well without scandal or kerfuffle, and has an independent career and source of income. Best of all, she would bring additional exposure to the EU for Andrew, which would be spectacular for his role as trade ambassador.