If you are satisfied with the introduction to Eilistraee, I'm fine with it. I'm not particularly into online RPing, but I love Eilistraee, and since you wanted to learn more about her through RPing, I felt compelled to help. I hope that it was helpful and entertaining (I had quite a bit of fun with this myself).
If you want to timeskip and make a story based on one of the characters, Cassia's past lends itself quite well to introducing threats. I don't know about Sevran's backstory, but while Phaere is an old NPC of mine, she is rooted in a world that is very different from FR (although Eilistraee's there as well), it would be hard to work her plot hooks in. I just have fun playing her, so I used her for this. Nimue, Viserys, and Valia were created on the spot, and they only have a very loosely defined past and background.
As far as I'm concerned, it's up to you and Bhaern.