Settings using Infernal War Machines?

[Sparky McDibben]

Suggestion: instead of spell slots, which only some people have and can make the provider feel like a glorified taxi driver who can't actually contribute beyond transport, use hit dice. Everyone's got them, it conceptually maps really well (powering it with your soul energy), and it doesn't directly interfere with normal operations.

You could also go 3.5 with it and use XP.