I don't think quality gameplay is dictated by a war room. Advance Wars and Fire Emblem, despite their superficial similarities, are just very different games. Fire Emblem has superior gameplay in some regards, while AW has superior gameplay in others. You can't just say one series has better gameplay than the other.
I don't know that cramming an AW-style war room into FE would really help enhance FE's gameplay, or just make it more like AW, which I would not consider a good thing -- if I want AW, I'll play AW. If they can find FE-styled features to put in such an option that would make it remain consistent with FE gameplay, then sure, go for it Intelligent Systems. However, if it's just there to satisfy people who'd rather be playing Advance Wars, then no thanks.
As to longevity, FE has it in spades. Immediately after I finished FE7, I started a new game with it -- thanks to the game kindly providing an entirely new scenario to play through, with new characters, maps, higher difficulty, and new and revised story sequences. If Fire Emblem DS brings that back, and perhaps adds in another scenario on top of that (three full lords rather than two full and one partial), then I will be very, very happy.