The Demon's Surrender question

I am a huge fan of this series but i found myself really disappointed with the ending between Nick and Mae.I loved that the third book was written from Sins personality she is a really amazing character and i love that Alan got his happy ending i just felt there was so much more that could have been done with Nick and Mae.
The third book and the series is great and i would recommend it to anyone who loves fantasy. However i feel the ending between Nick and Mae is really underdone especially as we watch as Nick battles with himself to become a better person for Alan and by the end of all three books Jamie and Mae also. We also watch Mae battle with trying to stop herself falling for someone who may never love her back, but by the end of the series Nick knows Mae means something to him, he may not be able to explain it but she means something to him and Mae knows she is head over heals in love with Nick. After everything they went through i feel that their ending was really disappointed because you don't know what will happen next with them like you can guess with Alan and Sin.