Hm, true, but all of the current Dukedoms (Besides Aetoria) were created due to the Wars of Unification, either as a reward for being a powerful lord who supported King Edwin during the wars (Cunaris and Havenport) or a powerful lord who peacefully submitted to the UK’s rule after the war (Wulfram and Warburton). Creating a new dukedom as a form of reward might not be accepted by the Tierran aristocracy.
It has been explicitly stated that whoever marries Katarina will get the title of her earldom. I’m not sure about Eleanora, but it has been mentioned that she is an orphan and there has been no mention of siblings, so that is probably the case with her. For marrying either, there would be pros and cons:
Definitely comes with a title.
One of the richest titles in the Unified Kingdom.
Comes with a lot of political connections and influence.
Almost certain strong resistance to a marriage between her and a poor baron.
It would probably come with a lot of influence over the MC by Katarina.
It has been insinuated by @cataphrak that continuing to follow Katarina (and Caius) will lead the MC down a dark and morally ambiguous path in future installments. Trying to create a successful relationship and marriage will probably contribute to this. Though if that doesn’t bother you, then you should be okay.
Less resistance to a marriage (Compared to Katarina).
Probably has a title attached.
Comes with a fair amount of financial security.
Eleanora has very strong feminist and egalitarian beliefs (Such as removing the purchase of commissions and instituting exams for the army) that she champions, and would probably expect her husband to join her. Tierran society is very resistant to such reforms, which might damage your reputation. Though if your MC has already expressed such views, then it will probably not be as big of an issue.
There is also the viscountcy of Wolfswood. The house has no heirs left, and might adopt someone into the family and title (An example of this happened to one of Jane Austen’s brothers). If you give enough support to the Viscountess Dowager in supporting the sainthood of her soon, this could happen to you. Or if you marry into an earldom and have a brother, then maybe your sibling can be adopted into the family and title.