If the future George IV had been made to marry earlier (1780s rather than 1790s) what princess may have be chosen for him as a bride?
I want to say that we can rule out Savoy due to religion. Stolberg-Gedern doesn't seem to have been anything more than a princess and wouldn't be important enough to attract a match with the Prince of Wales. Württemberg is likely already promised to a Russian match.
Augusta Wilhelmine of Hesse-Darmstadt had delicate lungs and might not appreciate British weather. Caroline of Nassau-Usingen is a grand-daughter of George II so I wouldn't be surprised if she were considered. Juliane of Hesse-Philippsthal seems like an interesting woman. Certainly she appears to have been more competent than George IV himself. If you can get Louise of Hesse-Darmstadt from the match with her cousin, the crown prince of Hesse. Then George might very well be pleased with her. I suspect the man needs a beautiful and fairly agreeable wife to be happy domestically. As well as someone to keep him from straying too far with mistresses.
Karoline of Nassau-Üsingen: again, besides Nassau-Üsingen (as well as Hesse-Philippsthal) perhaps being too small a player to make an alliance worthwhile, I can't see an impediment. @Titania's Rose she wasn't George II's granddaughter (different branch of the house of Nassau - namely Nassau-Weilburg - descended from him).
Sorry to bump this but this topic has been on my head recently and I though bumping a recent thread would be easier than making a new one. What do you all think about Princess Luisa Wilhelmina of Orange-Nassau (28 November 1770)? As the daughter of the Stadtholder of the Netherlands she seems to be one of the highest ranking potentials I can find. Britain and the Netherlands were traditional allies and the two were cousins, both descending from George II, so on paper the marriage seemed perfect. OTL Luisa married Karl Georg August, Hereditary Prince of Brunswick-Wolfenbüttel, a match arranged as a way for the Oranges to show their appreciation for the Prince's father Karl Wilhelm's divisive role in the Prussian intervention in 1787 that restored the Stadtholder to power. Considering that the Prince was physically and mentally disabled the marriage was quite a waste. So lets say the Oranges make a different marriage with the Brunswicks (the Hereditary Prince of Orange and perhaps Caroline of Brunswick?) or don't feel the need to make a match, leaving Luisa single. Could she become Princess of Wales in this scenario and would this be a better and more fruitful relationship than that of George IV's OTL marriage?
Hadn't the Netherlands recently fought against Britain in the ARW? It might take some doing to get Parliament/the royal family to stomach a Dutch match (although George III's daughter Mary was considered for either Willem VI or his brother, Prince Frederik). It sounds like a decent match - but I'm not sure how plausible it would be
As far as the state of the marriage goes, everything depends on when it happens. If somebody can (a) get to the prince in mid-1783 before he meets Maria Fitzherbert and (b) arrange it so he thinks the marriage is his idea, it will be rock-solid. He won't be faithful to her, but he won't be antagonistic either and will probably manage to have more than one child with her.
If it happens a few years later and they yank him directly out of a relationship or unrecognized marriage with a woman he loves and force him into a marriage with a stranger (the first time Parliament relieved his debts in 1787 might be an occasion for this) things might actually be worse than they were with Caroline, if that's at all possible. The one positive is you wouldn't have a royal mistress giving "advice" to the royal bride.