Upon watching Red Faced (the 6th episode of the Supergirl series), it suddenly occurred to me that there is a possibility of Supergirl one day donning the red ring in the series.
As mentioned in a previous episode, Supergirl has ventured off of Earth and visited other planets. Although she did so to be able to say that chocolate pecan pie is the best desert in the galaxy, she has still visited 10+ planets aside from Earth. With this in mind, it is possible that she could eventually meet a lantern whether it be Green or Red; an interaction that could set her on course to subsequentially donning the ring of rage.
Rage has been very present in the last few episodes with our dear Kara. In the latest episode, she is seen losing her cool multiple times and is furthermore labeled as having anger management issues. This could simply be a micro plot that was used to aid in the effect of that single episode, or it could become a more overarching theme that will be repeated through her character trajectory.
Although this is all currently far-fetched speculation, it may still hold some credibility given the current elements of foreshadowing that are being highlighted in the Supergirl series. Furthermore, I would love to see Kara become consumed with revenge in some manner; maybe the loveable snarky Maxwell Lord may do something to her sister to trigger the bloodlust that the Red Lanterns are known for. For now we will wait and see if Kara one day becomes consumed with ...