Longer/less common (and more normal) middle names:
For uh-LAY-nuh:
Elena Jubilee
Elena June
Elena Jacqueline (all of these J names go so well)
Elena Martinique (VERY random, but goes nicely)
Elena Mattie
Elena Isobel
Elena Katrine
Elena Sophie
Elena Angeline
Elena Bree
Elena Juliet (Or Elena Jules)
Elena Jeanine (I hate this name because of a character from a book but it sounds really pretty)
Elena Jean
Elena Melody
Elena Natalie
EH-leh-nuh middle names:
Elena Jubilee
Elena June
Elena Joy
Elena Jacqueline
Elena Martinique
Elena Angeline (I Love it with this pronunciation)
Elena Cove (Saw Cove on another thread)
Elena Lise
Elena Bree (insert heart emoji 
)
Elena Juliet (Or Elena Jules)
Elena Jeanine
Elena Jean
Elena Bridget (love love love this name)
Elena Violet