Flamethrowers were extremely possible in RTW, just by changing properties on the missile and generating a particle trail. Unless they have really changed things, I don;t think there is a need to have the expansion just to do this

I can look at the possibility of things like the Screaming Bell ... it would make an impressive sight on the battlefield.

Also...as to unit 'balance' I envisage the Skaven as having large numbers of cheap units early on, which would make them a dangerous force in the early game. they lack quantities of heavy troops....so as the other factions develop, they will find themselves increasingly pressured. Development of firearms and heavy weapons will be their necessary survival strategy, along with the reliance on assasins and spies to undermine the enemy. Storm out, fight hard early on, and hang on! They would also have to rely on multiple waves of armies to wear down enemy troops who would take longer to replace losses. Battles of attrition would be the key. I think they could be a very interesting faction to play!