OK I've read through this a bit and
@Harriamelia and
@Valena , I feel that you two are way off marriage-wise. First off, James I saw himself as a peacemaker, a bridge between the Catholics and the Protestants. For that reason he wanted Elizabeth to marry a Protestant and his heir to marry a Catholic. So there's no way in hell that James I would consider a second or third rate German power for his son and heir when daughters of Spain and France are available instead. So some kind of Catholic bride is highly likely, unless you handwave James I to die shortly after Elizabeth's marriage. As for Catholic brides, there's the (in)famous Spanish Infanta (which is pretty out-there but not impossible), Elisabeth, Christine or Henrietta Maria of France (if the POD is before Henri IV's death, as the Spanish negotiations were done under Marie de' Medici; plus not sure when the Savoyard talks began) or Medici/Savoyard Princess (both were offered at one point as proxy brides during the Spanish Match).
In my opinion the best bet would be a marriage to Madame Royale Elizabeth, with a POD of a surviving Henri IV. Henri was a former Protestant, so he's someone that the English could do business with; and Elizabeth would be the closest in age. As for the other bridegrooms, we'd simply move Christine to Spain and Henrietta Maria to Savoy. Henry Frederick wouldn't be happy but I can't imagine that he would openly tell his father no and open up a major domestic crisis in Britain.