Modern Elves is so well-polished that it seems nothing could make it better. But what if you added Incubation Druid to a list with Oran-Rief, the Vastwood and Llanowar Reborn to put the +1/+1 counter on it as early as possible?
Since Elves strongly relies on fast ramp, it looks like Incubation Druid would be a perfect addition to the deck. But of course, it could be used in the same way in any other deck that relies on quick ramp, including many combo decks.
Either way, Incubation Druid is definitely a worthwhile investment for any serious Modern player.