I agree with you, but the way things tend to work out in practice is that armies that have recently been worked on are often more likely to get more content soon compared to armies that haven't darkened the desks of the design team in 5+ years. It's the classic "we made a ton of stuff we'll dribble out in waves over the next couple of years" vs "Wait, we still make dwarfs for AoS???"
To me, CoS feels like an obvious first wave that will be added to before long. FEC, I dunno, I'm not really expecting anything major there unless their battletome hints at a lot of stuff or something, I wouldn't know.
Fyreslayers have actually been getting a lot of attention lately, I fully expect they'll get at least a small wave of new models in 4E.