As for ideas, I think I could work with Machanist. As mentioned the party will need a form of scout - I'll work out how best to fit it into the universe, but it's very likely you'll be using trap components from disarmed and dismantled traps to create machines and constructs (which will do nothing on their own and not exhaust you of any energy unless it's something BIG) - You'll then be able to weave magical runes, symbols and seals onto them which will "program" them with life.
You could create small mechanical creatures that can scout out rooms, suicide bots to trigger traps and more. You'd work well with the rogue - you're both mechanics and if you encountered sufficiently difficult traps, locked doors etc. you could both team up to handle them. You'd work well with the Geomancer, at a sufficiently high level you could exhaust an immense amount of energy to give a short-term sentient life to a rock golem of his creation. If the chanter works his magic on your components you could create a machine resistant to an element or with low-powered healing.
Finally the Machinist's weapon - Yes, they wield guns. No, they're not high powered rifles or machine guns that spray super-damaging bullets all over the place. They're just like the rest of the Machinist's creations: Constructed items that are imbued with runes and symbols of power. They work as "charged" weapons, the Mechanist uses his mental power to inscribe a rune or symbol of power upon his gun similar to how a wizard would cast a spell and when the power of these runs is expended the weapon becomes effectively useless.
Machinists will be extremely weak combat classes when directly confronted in combat. They're not physical combatants. They're not magic powerhouses. They're tinkerers. They make and design traps, they're the people who design the mechanisms that link to a drawbridge. They will become a different kind of caster entirely. A wizard will cast a fireball. A machinist first needs to find mechanical components. Usually from deconstructed traps. They must then spend a round or number of rounds using their mechanisms to construct their creation. They then carry these inanimate objects around until they need to use them, at which point they then spend a round inscribing runes and symbols imbued with power onto their creations. FINALLY they can use their creations.
But you can't use a wizard's fireball to scout, it won't neccessarily be effective at triggering traps, it can't be used as a sentry either.
We'll avoid the Steampunk "I wasn't aware boiled water could form alliegences" style of mechanists. They're more like artificers really, but artificers deal with creating magical items more than anything else. Still, if this is something we can all agree on (as I'm certainly not entering my final decision here) I'll get to finishing off the last two character sheets. Eventually!