Well theme is a curious beast to tackle but I still think it can be done if we keep things a bit open ended. LOTR LCG is supposed to be taking place before the actual story of the books, so that there is leeway to deckbuilding and questing. Similarly a First Age expansion could have thematic leeway by being not exclusively recreating the stories, or by presenting them in a different fashion (e.g. being retold as tales). There could even be a seperate First Age box that is individually playable out of the box (like its own little core set) but still plug-and-play with the other expansions. After all whether a player takes the Eagles inside Moria or not is a personal choice, not a weakness of the handling of the IP imo.
Lotr LCG newer expansions sell well in my area still.