Only tangentially related to Supergirl and the Arrowverse, but the CW has ordered a magic-based gang drama from Rob Thomas directly to series. Rob Thomas is the executive producer of iZombie (as well as past shows like Party Down and Veronica Mars). Thomas is also developing an adaptation of The Lost Boys for the CW.
This is relevant because it shows that the Warner side of the CW is focusing on specific creators. These other adaptations from Thomas are good leverage for iZombie getting more than just one more season after this one.
This is another reason why none of the Arrowverse shows are going anywhere -- CW has a lot invested in Greg Berlanti. Arrow, Flash, Legends, Supergirl, the upcoming Riverdale, plus the CW Seed shows Vixen and Freedom Fighters: The Ray all add up to a ton of pull for Berlanti with the network heads.