I love Mila and Luca. I like twins names to have some kind of common "theme" or thread, but nothing terribly obvious. I think that Mila and Luca are perfect because they both have 4 letters and end in the "-uh" sound. To me, Danica and Luca aren't tied enough, and I personally don't like the name Danica (or Sebastian) much. Good luck and congratulations!