I'd go d10 system, but success base not percent. Modeled more after the World of Darkness method, so there wouldn't be classes per say, but Archetypes that a player can model their character after to achieve a sort of balance of class.
Yes, totally agree with it. WoD like d10 mechanic usually serves for a good balanced roleplay game that allows to concentrate more on the "roleplay" part of the game instead of constant throwing dices and searching tons of useless tables for rules like in D&D. And most f10 system are easy to convert on other setting.
In fact I know even one RPG game that is quite similar to the world of Guns of Icarus. It's called Crystalicum and it's in fact a crystalpunk (i'm not sure if such a word even exists but it's the best way to describe it) RPG based on the world from the movie "Treasure Planet" but I guess steam-powered ships flying in the skies are not far from crystal-powered ships flying in space. It's also a d10 mechanics easy to alter.
Of course we could also take some steampunk-oriented RPG and base GoI RPG on it. Or some system that's about having a ship and a crew (I'm sure there are some). What I mean is that a similar in assumptions RPG system as a reference would be good.