The way I see the races in Iron Kingdoms is pretty straightforward conversion. The different human ethnic groups would have their optional ability adjustments, gain skill bonuses in the equivalent RQ skills at +5% for each +1 bonus, and gain +1D20% in what is listed as their automatic class skill. If these benefits are taken, they also take the character flaw as well. Base weight determines modifier to their SIZ.
(This means I need to determine the effect of these character flaws. The ones that are mentioned are Jingoistic, Socially Isolated.)
The character classes work out as follows:
Barbarians is just a label applied to certain people. There is no real distinction between "barbarians", "rangers", and "berserkers"; it's just how you look. The class restrictions do not apply.
Bards are described as sorcerers who take (or are forced to take) military training to legitimize their abilities. I could make a Warbard advanced skill; success means you grant a +5% bonus to your allies' attack rolls and increase their damage dice. (As a general note, I think it useful to have a way to increase dice type rather than grant damage bonus, given the way hit points work in runequest)
Clerics would be called "priests" and are ones able to use divine magic. Turning Undead sounds like an advanced skill that is only useful if you have divine magic. I never liked the Pain of Healing rules, and alignment does not exist in Runequest, so in IK-RQ these only apply when priests of certain cults try to cure adherents of certain other cults.
Druids deserve detailed treatment. My idea is that druids with abilities matching the D&D class exist for the cults of Dhunia and the Devourer. Other cults have priestly orders dedicated to handling nature, according to that deities goals; these would include "shamans" and "antidruids".
(For those not familiar with Iron Kingdoms, Cyriss is a goddess of mechanical stuff, and given her description, a kind of antidruid seems apropos.)
Fighters are common. It's easy enough to define soldier professions. (Well, it's not easy enough, IMHO, but you get the point.)
Monks exist in one of two orders in the Iron Kingdoms. The monk abilities in D&D can be converted to Legendary Abilities. It would make sense to have two different sets of Legendary Abilties, one for each order. Differing monk "professions" and weapon skills also serve to distinguish between these two orders.
Paladins are fighters who use divine magic. Since alignment does not exist in Runequest, there is no reason why divine fighters can not be defined for each of the cults, although few cults would recognize divine fighters of other cults as "paladins". (Menoth and Merrow would be a rare instance of a pair of deities who recognize the "paladin-ness" of each other's paladins; I'm not sure what kind of effect this kind of relation would have, given that Menoth and Merrow are fiercely competing for the same worshippers.)
Rangers Since Runequest neither provides anything to distinguish a "ranger" from a "barbarian", nor provides a spell progression for rangers, there is no need to define a special ranger class for the Iron Kingdoms. Some rangers may cast spells, if they gain spellcasting as anybody else may gain it, through what in D&D would be multiclassing. (An example would by my Nyss character Sorceress/Wizard/Ranger.) Since runequest has no multiclassing penalties, there's no reason there can't be ranger/paladins of any cult. Even paladins of Cyriss could study the wild in order to better exploit it, although such characters would probably not be called "rangers".
Rogues are common in the Iron Kingdoms, being simply people with rogueish skills and attributes.
Sorcerers could be handled in different ways, as noted above. I need to see the Runequest Companion before I can decide on the best way to handle them.
I've always seen my Nyss sorceress as just like a wizard, except without a spellbook. That's changed once I multiclassed her, except that multiclassing sorcerer and wizard sucks in D&D. Runequest does not have that problem, so the only question is whether sorcerers use the same kind of magic as wizards, or whether it is something different.
Wizards have a skill called Read Magic, which allows them to read and write spells in spellbooks. Runecasting is still needed to cast the spell. Since runecasting seems such a natural fit to the Iron Kingdoms, there seems to be no reason not to use it for wizards.
Iron Kingdoms also has additional advanced classes:
Arcane Mechaniks have the Read Magic skill as Wizards do, as well as Arcane Mechaniks skills.
Bodgers have the "Bodgering" advanced skill, as well as Arcane Mechaniks skills, however often lack Read Magic and Runecasting skills that are typical of the Arcane Mechanik. Of course, nothing in Runequest prevents characters from mixing and matching skills as desired.
Bodgers would also have a selection of Bodgering Legendary Abilities, such as "Toss a Hammer At It" (page 99 of the IK Character Guide).
Fellcallers are "anti-bards". The Fellcalling advanced skill is only available to trollkin. Fellcalling costs Manna (which is simply a term I'm using for Magic Points, since I prefer the way it sounds), so to get more fell calls a day, a fellcaller needs more Manna.
Gun Mages have the Gun Magery skill. They also master the Gun Rune, and can use Runecasting. Gun Magery allows the spells to be cast through the gun, which is quite a handy trick.
I'm not sure if gun mages should or if they should not be allowed to read and write their spells using Read Magic; if no system of writing gun magic down has been developed, then the Read Magic skill would not be possible for these spells. OTOH, I can't really think of why or how these spells could not be written the same way any other spells are written. Perhaps because the Gun Rune was so recently discovered, very few casters, if any, have figured out how to write these spells down, and those who did aren't sharing their secrets.
This should give people things to think about...