I could see Chakotay as a Science Officer.
Seska leaving Voyager for the Kazon never made a great deal of sense to me except that once Our Heroes knew she was a Cardassian she didn't really have anywhere else to go.
If Ro's actually a Cardassian sleeper agent who somehow gets activated during the trip, it makes sense to me that her arc might follow Seska's, though it might be more satisfying if her experiences as Ro led her to realize she could be a part of Team Good Guys if she wanted to be. OTOH, I admit that in Star Trek stories it sometimes bores me when it seems that Our Heroes can find an understanding and/or common ground with almost anyone. Also, if Seven still joined the crew then her own integration with the crew might very much have a feeling of "been there, done that" at this point.