Queen Louise of Denmark and Princess Alexandra both hoped that Frederik IX could marry either Louisa or Helena. Alexandra was rather distressed at Victoria's choice of Prince Christian, and refused to accord him the respect of a duke of Schleswig, since "those duchies belong to papa (Christian IX)".
Christian OTOH actually thought that the summons to England from Queen Victoria was because THAT lady was looking for a new husband
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Out the way with that then, Helena will be married to suitably Protestant, suitably important German monarch/erbprinz. Prussia's out. Hannover Victoria couldn't stand. Hesse-Darmstadt's out. Kassel's too Danish. Saxony and Bavaria are both Catholic. What about Baden or Württemberg? Oldenburg might do, now that I think about it.
Unless Victoria can convince Willem III of the Netherlands to agree to take Helena as his daughter-in-law/second wife. Though considering Victoria's disdain for the Dutch royals' appearances, she might need a brain transplant.