Rhi

Thranduil and his father Oropher are meant to fight in the War of the Last Alliance. That won't happen until season 5, but it would be good to be introduced to the two characters before that. The downfall of Eregion and the loss of so many elven lives should be enough to gain their attention, and I can imagine there will be tension as they didn't get one of elven rings - nor were they informed about the rising threat of Sauron.

I don't know if we'll see Legolas. Tolkien never gave Legolas an age and it was only when the trilogy came out that Legolas's age was put at 2931 years old, so he may not be born yet. Since that was a non-canon age though, it can easily be disregarded. Personally I'd like to see a child/ young Legolas. It'd be interesting to see him left out of the Last Alliance because of his young age as this would mirror the reason that Gimli was left behind during the events of the Hobbit.