Will Moiraine either Want or Need a new Warder...do you think?

Thom makes sense. As for the no foreshadowing remarks... She asked Thom to come and rescue her. She did not ask Lan or anyone else. The fact she asked him to go after her means that she already sees him as a Warder. Going after the Aes Sedai when she is in trouble is one of the primary responsibilities of the Warder. If Thom is in fact successful in the rescue then he will have both proven his usefulness and have earned the trust she placed in him.