I agree, for the most part. The northern portions of the Papal Legations would be one's best bet (maybe also including Forsi-Cesena), since they are relatively inoffensive to even the most Catholic nations of Europe. And I also agree on the 100 Days assessment, since that is what killed the possibility of Joachim Murat keeping the Kingdom of Naples as well, so Eugene would also definitely have better chances if there is precedent like Murat.
Forli' and Cesena are in the Legation of Romagna.
There might be an issue in having both Murat and Eugene (outside of family, they were possibly the closest to Napoleon) as rulers of Italian states which happen to be contiguous, but I believe that it might work out if the 100 Days do not happen (which may result in the Coalition powers be more divided into two antagonistic lineups in Vienna since the scare of a Napoleonic return does not materialize: Russia/Prussia vs. GB-Austria-France. When the Congress was adjourned in December 1814, the former Coalition was not a compact monolith). IMHO it will depend on Alexander's assertiveness (Metternich was quite frightened by the czar's proclamation that the downfall of Napoleon had happened "in the name of liberty and enlightenment" and both he and Castlereagh were fearful of exchanging the former French domination of Europe with a Russian one).
A Beauharnais holding the Ionian Islands? Might that make him/his dynasty a front-runner candidate for when the Greek throne is up for grabs?
Alexander had in the past made attempts to establish a Russian presence in the Ionian islands, and in Malta too. The idea of Eugene ruling the Ionian islands will certainly meet a firm British opposition.
At one point he was offered the Duchy of Milan.
That'd make for an interesting TL.
Never heard of such an idea, do you have any link?
I believe it might be very difficult, since the duchy of Milan was a possession of the Austrian Habsburgs since the war of Spanish succession. It might be (slightly) easier to give Eugene the duchy of Venice (probably without Dalmatia and the Ionian islands) but for that to happen Austria must retain the former Austrian Netherlands.
When the Swedish heir was being elected Napoleon was still winning big. And even if he was losing I don't see Eugene abandoning him to go and rule Sweden. Ionian Islands do sound interesting but maybe somewhat...(trying to find the right word)...exotic. Even so, though I'm not sure how many butterflies would be necessary the Ionian Islands and later Greece sound great
But Pontecorvo, Lucca, Parma or maybe a new Duchy carved out of Papal States all sound very plausible and also could have big consequences in Italy. IIRC he was still holding the line in Italy when Napoleon lost in 1814 and then after the news came Napoleon abdicated Eugene tried to get the crown for himself but didn't have support. Can anyone think of a POD for this to work? Not the whole of Italy but at least the allies agreeing to give him a slice of Italy for himself. Which part, that depends on the Congress of Vienna.
Pontecorvo is a joke: a landlocked, postage-stamp-size principality completely surrounded by the kingdom of Naples: I don't think Eugene would be interested.
The other possibilities are more real. The POD IMHO can only be no 100 Days and a more assertive Alexander I