Chaplain: You probably want to not include chaplains, what with the more logical, faithless elements of the source material. Alternatively, you could keep the stat line and treat the chaplain as some kind of champion of Ferrus, who has received some of the C'tan powers or abilities to represent his proximity to the "ideal" state of mechanization
I would like to adhere to the traditional IH fluff of the Iron-Father here - that is my vote.
Librarian: Take out obviously. perhaps replace with some kind of Uber-pariah character?
My thoughts too (see my post above). One thing I think all are in agreement on here is not doing the "one-for-one adaptation (either turning Necrons into Space Marines or turning Space Marines into Necrons. I think the list would be more flavorful if we adapt more ideas in a similar manner to this one.
Dreadnought: A hybrid of the Tomb spyder and the dreadnought would be interesting, especially since you can easily take the tomb spyder's arms and substitute them for a dreadnoughts close combat weapon. since the Necrons don't have any template weapons, keep the flamer and heavy flamer options, but you can also allow them to take the gauss cannon or the Heavy Gauss cannon.
Here, I think we are risking doing doing just what we want to avoid. The flamer weaponry will have to go (for game balance and consistency with the fluff). All ranged weaponry on the Dread will have to be gauss-flux. I think the "we'll be back" concept should not apply to the IH army (again, we should not be trying to turn Space Marines into Necrons), but applying it to the Dreadnought might be fun.
Terminators: making terminators a troop choice is a very interesting idea. the only problem is you are forcing the army into a very small size that plays against you if you include the phase out rule. instead of storm bolters, they should carry the Gauss blaster normally seen on immortals.
Terminators become very rare constructs in this alternate IA - only seen leading squads in special missions/detachments. I was thinking of treating them similar to the current Techmarine - a standard profile IC that occupies an Elite slot.
Tech-marine: it might be interesting to treat the tech marine like an apothecary in that he grants feel no pain. see below on the tactical marines.
This could be an adaptation of the Iron-Father or be omitted in favor of the Iron-Father.
Scouts: I don't really envision these iron hands fielding scouts as snipers in the traditional manner, so you could take a page from the space wolf codex and make them an elite choice, full marine stat line unit with some form of gaus sniper rifle.
I've always had trouble visualizing Scouts in an IH army - I am at a loss as to what to do with this one. Maybe omit them altogether? Remember, this army is on the "bad-guy" side in the alternate IA - Chaos Space Marines have no Scouts, why should the IH? In facct, the core units should be patterned more closely off of the Chaos 'dex instead of the Space Marine one.
right now, vanilla tactical marines emphasize felxibility over specialization in shooting, cc, etc. Necrons warriors emphasize durability and shooting, with their lack of grenades and we'll be back rule. To keep from making tactical marines just warrior clones, you can do two things. First, pare down the number of upgrades available to their sergeant and special weapons available to the squad. Second, if we just give the tactical marines the "we'll be back" rule, then they are going to cost around 20 points a model!!! Instead, I think a good solution is to have them receive the "we'll be back" rule from another model that join the unit or is nearby much like how the resurrection orb item from the necron armory works. I would choose the like the techmarine, and possibly make a master of the forge HQ choice grant a similar but more enhanced version. That way, if the techmarine goes down, then the unit is easier to take down and thus more balanced.
Tactical Squads (as the core troop choice) will have to be treated very carefully. the balance of the whole army will depend on it.
To be consistent with the IA, the unit will have to have gauss weaponry - period. That will mean a higher base cost. I agree that the Sergeants options should be reduced (maybe even eliminated). To avoid the unit being Warrior clones, perhaps we should use the other Necron weapons as special and heavy weapons for this unit?
Bikes: Contrary to common belief, necrons are not slow. They are often played slow because of an over reliance on foot slogging warriors, but units like destroyers and wraiths are jet bikes and can be very fast in their own right. you can easily adapt that idea to represent Iron hands that have been mechanized into half-marine/half-jetbike warriors. Once again, you can substitute gaus weapons for the bolters fairly easily.
For a real treat, instead of using the pariah units as the standard foot sloggers, have them be mounted on bikes wielding those nasty warscythes. Gaus lances? I need to think more on that...
I think this might be turning Space Marines into Necrons again. Maybe using the Necron technology to develop a new style of "jet" bike?
Assault marines: Once again, I don't think the Iron hands all have to be slow. You can keep the assault marines in, but consider re-envisioning their role. Maybe gauss pistols instead of bolt pistols? replace the chain swords with some kind of anti armor weapon, like the one used by the tomb scarab swarms? Also, to keep the feel of the necron wraiths, I suggest stealing a page from the eldar codex and making the assault marines ignore difficult terrain. If those silly harlequins can do it with their anti-gravity belts, why not have iron hands who can simply phase through the terrain?
I think treating them more like Warp Spyders is the way to go here. Either that, or give them special teleport/portal rules instead of relying on jump packs.
Weaponry could include a type of gauss pistol or (possibly a game balance thing here), not allow a pistol gauss weapon at all. The members of the squad must choose between a ranged weapon (gauss) or close combat weapon (warscythe). It is a radical idea that would need some serious scrutiny - but I'm just throwing it out there.
Land speeder: on the one hand, land speeders are about as close to skimmers as most marines get. However, they seem like quaint toys compared to other races. I really don't see any use for the land speeder.
Land Speeders are post-Heresy developments anyway - so the IH should not have them.
Devastator: the problem with emphasizing devastators is that necron gauss weapons are...well...dull. there are only five types of gauss weapons in the codex and half can't be carried on foot. I guess you could have devastators choose between heavy and regular gauss cannons, but if you can put the same thing on a dreadnought, whats the point? if we are going to include devastators, they need to provide us with something we can't find on a dreadnought or another unit
Half of those weapons cannot be carried by Warriors on foot - who is to say that Space Marines can't carry them? I say give Dev's the heavy gauss weapons. It is a good way to distinguish them from Necrons.
Landraider: I agree with the earlier sentiment, you can't slap a portal on a land raider and guass weapons on the side and not call it a monolith. variations is key, and sadly I don't think you'll find much of that in the current necron dex.
Do we want all Land Raiders to be so equipped? Or just "special" ones?
Predator: I personally, thing you should turn the predator into a skimmer. It makes more sense for it to be a hybrid mini-monolith since it will have to fulfill the hole left by the complete absence of razorbacks or land speeders.
An interesting idea. Maybe a "skimmer" upgrade for any vehicle - like extra armor?
In summary: I prefer army lists that have some flexibility of choice rather than pigeon hole lists that only play well one way. I think that there need to be some caution here because the necron codex really pushes towards the latter, whereas the current space marine list heads toward the former. if you include terminators as troops, then that doesn't mean people will take 6 units of termies as they won't have the points for it, it just means they have fewer options for troops. If the troops have fewer options for upgrades, same problem. If you replace all the space marine weapons with gauss weapons, then you have fewer ways to differentiate dreads, devastators, and predators. The solution probably lies in different ammo and new home made weapon types. Gauss sniper rifles, gauss flamers, gauss assualt cannons. etc.
I agree that meshing the two concepts should be deliberated at every point.