The more I think about making a playable game out of the Mortal Engines books the closer I come to 3 effective methods:
I) City on City - tiny scale cities, each about the size of a 1/72nd tank (maybe with big cities up to the size of a 1/35th scale tank) scooting around eating each other, with airships etc out of scale (well, seeing as they are larger, what with being closer to the viewers eye) but present for landings and what not. In terms of game play, it'd be rather like a land version of a Space-ship battle game like Full thrust... infact, FT might not be too bad an idea for a rules set. This is very definitely limited to the later books, as there are actually moving cities. I guess something similar could be achieved with land barges and traction fortresses, maybe evening making them vaguely inter-compatible by doing so in say in 3mm or 6mm. Certainly some of the Epic Ork models would work well.
II) Skirmish 'Gang' warfare inside a city - Effectively Necromunda or combat zone, involving various 'gang' size warbands skirmishing in a multi-level diesel-punk industrial setting. There are quite a few suitable figures out, and games would be along the lines of the Engineers guild try to storm London Museum, or a small band of anti-tractionist terrorists battling with say the crew of a pirate suburb. It could also work for the early period stuff - A small group of London Trained Bandsmen skirmishing with Movement warriors in the outskirts of London, or what have you. No vehicles or suchlike used, with the focus very much on each individual figure.
The final option (III) is really limited to the Early period (pre-traction) stuff, which is the 'massed battle' game. Although there are possibilities for Battle Mammoths, Campavans, traction bunkers and the like in 28mm, unless one is using a particularly large board, they cannot be used in any real numbers; and certainly for the likes of traction fortresses are really relegated to the role of mobile scenery. To actually be able to deal with a reasonable number (say a few warmachines and a platoon or two of infantry) will require a smaller scale - around 10-15mm. Ruleswise, maybe GASLIGHT, or a varient...