Approaching the redesign of a faction is a challenging concept, typically GW do the models first and that allows the rules designers to know the constraints of the faction and can build the battletome around a set list of models that are coming soon. This allows a structure that when approaching rules first can... lack, where anything is possible it mistakenly leads to the idea that EVERYTHING is possible, so breaking down the unit options as follows here is what I currently see us as having:
Duardin Engineer (Existing Model)
Gunmaster (Existing Model)
HERO A (Monsterous, built as secondary option to a different kit)
HERO B (Cavalry or flier)
Riflemen/Arbalesters (Dual Kit, Battleline if Ironweld) - Working in well with the Gunmasters,
Phalanx/Something (Dual Kit) - Here I'm considering factoring in the Cog soldier aspect, potentially functioning better when in the presence of an engineer essentially having the Duardin Engineer acting as a buff hero for the Cog aspects of the army. It would rob the labourer aspect of a second unit to benefit from faction specific buffs.
Ironsworn Templar (Also Builds Paladins Hero) - Non-Monster heavy hitter unit, monsterous infantry.
Ironsworn Fusiliers/Lancers - Fast Moving Cavalry
Gyrocopter/Bomber (Existing Kit)
Helstom/Helblaster (Existing Kit)
Duardin Cannon (Existing Kit)
Steamtank (Needs a new kit, likely a dual kit. My theory was an Iron Dragon rocket spitting tank, Steel behemoth labourer carrier)
Taking that as a base it means that new kits wise it would be:
Weld Riflemen (Dual Kit)
Weld Phalanx (Dual Kit)
Ironsworn Templar (Dual Kits)
Ironsworn Fusiliers (Dual Kit)
Hero B Clam Pack
Monster/Tank something kit.
Now thats already something thats rivalling the bigger releases, so I'd say at this stage to add a unit we'd need to remove one of my unit suggestions at each time (barring hero B which I have left spare currently). I think the best plan is add a unit, remove a unit policy (unless anyone can make a convincing counter arguement there
When approaching unit design I would go with the following criteria:
Does it fit the theme?
Does it fill a tactical void in the army? (Not every army needs to fill every void but the army should be competitive based on its own roster)
Does it bring synergy (Part of why I strive to add the Labourer keyword to Weld guard is to have interplay between them and the artillery in the faction, it gives a synergistic link. And just as artillery benefits from them, they benefit from Ironsworn nearby to inspire them. The benefits cascade down from having a mixed list.
Is it needed? (As above, not every faction needs to fill every niche, not everyone needs a monster or magic etc unless it forms a cohesive part of the army, theme and roster)
Is it something better provided by allies? (pretty self explanatory)
So what is the theme?
My suggestion is below, but this is simply my suggestion, it is entirely possible that my own interpretation is wrong or needs refining but here's where collaborative debate can help really hammer down the heart of the question "Who are the Ironweld?"
Industry Driven Humans and Duardin surviving a magical world through innovation and strength of arms.