This came up almost as soon as I posted my fell caller paragon path on the IK forums. Why did I make a paragon path instead of a full class for the fell caller? The fell caller was a full class in 3.5e, shouldn't it get a full write up like it did before? Well... no. The fell caller feels like a martial leader, and we already have the warlord. Also fell calls traditionally granted bonuses your allies and penalties to your enemies, and strangely enough most of the warlord powers do this too. It'd be far easier to use an inspiring presence warlord than to write up a complete new fell caller class, and in my games I do use inspiring presence warlords as fell callers - or the one I have in the adventure anyway. The warlord does lack a little of the spirit of the fell caller though, which is why I wrote up a paragon path for it. My favorite ability is victory howl, to me that feels very much like the fell caller, a nice balance between axe to face and the group gaining bonuses.
The gun mage on the other hand requires a full write up as it replaces the warlock as the arcane striker in the Iron Kingdoms. Previous iterations of the gun mage had three separate 'pacts': self-taught, Arcane Tempest, and Amethyst Rose; these reflected how your gun mage had been trained. Three separate paths however is a bit of a headache, instead I've come to focus on two different ideas for the gun mage powers: powers that you need a bullet for, powers you don't. Bullet powers use Dexterity primarily, with Intelligence modifiers applied to bonuses, and of course requires you use a loaded gun. Non-Bullet powers use Charisma with Intelligence modifiers doing extra things. I've come to think of bullet powers as raw damage and non-bullet ones as having that extra little bit of utility. Coming up with balanced skills however is a lot harder than it looks.
Arcane mechaniks and bodgers... I wasn't sure that bodgers needed a separate class write-up, but Volin's done a good job of it, once it's refined it should be an awesome anti-construct/mechanika striker. I recently came up with the idea that when you hit a target with the construct or mechanika keywords with the demolish at-will power you can use various encounter powers as triggered free actions. While this would be horribly overpowered in normal play, I feel it's suitably cool since it only works on two types of enemy. While I think the idea is awesome, Volin hasn't replied to my suggestions in his forum thread just yet, so we'll see what he thinks.
As for the arcane mechanik, I really like the idea of these as arcane leaders and doing strange things to the party's equipment that has the mechanika keyword/feature. I'll post my thoughts on how mechanika works in 4e at a later date, there's plenty of ideas I blatantly stole from other IK forum posters. Until someone does a large write-up though, I would use the artificer posted in Dragon 365 for the arcane mechanik.