This was my interpretation as well. She thanks R'hllor for his " pure white fire" in Storm of Swords. The only only other "white fire" reference I could find was in AFFC when Aeron is describing the glyphs on the purported Dragon Horn after it was blown. A Euron & Mel combination would keep with the fire and ice/dark and light theme. She also has access (through Stannis or in his place) to a ground force that Euron will ultimately need. Danny most likely, if given full information and choice, will not align with Euron. Thus, Mel may be necessary if Euron intends to take the Iron Throne or defend it against Danny. This is especially true if the Dragon Horn is fake or only can control one dragon at a time. If the Dragon Horn is real, no matter how many dragons it can control, and if Mel has, knows where or obtains the Horn of Winter, together they could be very dangerous and powerful.