I think that that Lucy and Gru are going to fall in love (Yes. It's bound to happen) and the villain for the film. I'm guessing that Eduardo (Al Pacino) will be like Gru because he has his own adopted child (Antonio) and he's not the true villain. If he's not the true villain, who is then (You may ask)? Well, the true villain should be Silas Ramsbottom (Head of the Anti-Villain League) which could mean that the League was set up as a LIE to hide his despicable schemes.
So, in the final act of the film, we should see Gru, Eduardo, Lucy and The Minions fight against Silas as kidnaps Gru's daughters and Antonio as the League should fall. As a result of this, I think that Lucy will stay with Gru, The Girls and The Minions as the film ends on a scene similar to the first one.