Well without advance paths, an alternate path is kind of a moot point.

But the concept is still valid, and a good way to show/allow a character to deviate from the norm or accomplish something that is not normally accomplishable like a Gland Warrior or Calixis Templar.

I think the right implementation is something like the elite advance package deals from Into the Storm. So you pay a hunk of XP and get a talent ot trait or implant not otherwise available, as well as a discount on a related set of advances.

So maybe something like:

Moritatus Accolyte

Prerequisite: WS 45 or greater, Linguistics:Secret Tounge Moritat

Pay 400 XP

You have been inducted into the secret cult of the Moritat so that you may better bring death to the enemies of the Emperor. As a believer in the doctrine of the Moritat you have a strong preference for edged weapons that cause bloody wounds.

You gain the trait "The Bloody Edge" as in the Inquisitors handbook page 31.

You count as having the finesse aptitude for the purpose of talents that also have WS as an aptitude.

When you reach 6000 xp, if you prove worthy you may become eligible to buy the "Moritat Reaper" package if you have remained loyal to the teaching of the cult. Be certain that they will be watching.

 That does seem like a reasonable idea. Although most alternate ranks really dont need representation.

Perhaps alternate ranks can exist as "partial" aptitudes? Where it makes a certain subset of skills/talents/traits cheaper by meeting a set of pre-reqs, paying an up front XP cost, and RP forcing the character down a certain path?

I would instead carry over the special abilities of the alternate ranks. Any advancements that those ranks would normally have access to would be available at a set price that isn't related to aptitudes. Similar to the way comrade advancements work. Ofc the PC would pay the XP required to get the alternate rank. This should prevent you from having problems with giving players new aptitudes, which would give them too many advancement paths and make it harder to balance the alternate ranks with the careers that already exist. You will be able to limit them to only certain skills that pertain to that alternate rank. That will also keep players from getting easy access to advancements that don't have anything to do with the alternate ranks they've taken.

 Ok we have DH careers what about RT careers?

Manyfist said:

Arch-Militant, Astropath, Explorator, Navigator, Void-Master, Seneschal, Missionary, Rogue Trader… Ork, Kroot, and Dark Eldar?

I'm also working on a Crusader spec. In the mean time, the WIP:

Space Marine Scout

Characteristic Bonus: +5 T
Starting Skills: Athletics, Common Lore: (Imperium, War), Dodge, Forbidden Lore: (Adeptus Astartes; Xenos or Heretics), Stealth
Starting Talents: Die Hard, Autosanguine, Light Sleeper, Resistance: Poison, Heightened Senses: Hearing
Starting Traits: Size (Hulking), Unnatural Strength (+2), Unnatural Toughness (+2), Auto-Stabilized
Starting Gear: Shotgun or Bolter or Sniper Rifle, Combat Knife, 2 Frag Grenades, Scout Carapace Armour, Cameoline Cloak
 

 Honestly for rogue trader.. hmm.. The Exploritor is really just the rogue trader version of the tech priest. Rogue Trader is closer to Black Crusade in rough XP value. Black Crusade is in between rogue trader and death watch but closer to rogue trader.

Unholy_Ravager said:

Starting XP for Guardsman in 600, for Specialists it's 300… So these guys would get around 1,200 to 1,500 and some really nice gear. The Astropath is throwing me for a loop, though. A Psyker with limited discliplines but more power?

 Ragh, trying not to get off-topic. Writing up the Crusader.

Should the Arbitrator get a Riot Shield? (+4 AP to body and held arm, +15 to Parry, -20 to attack with it but it does negligible damage anyway.) What other Specialties should I add?

 The list  of Specialities look good. For the Arbitrator i wouldn't give them a riot shield starting out honestly. They can aquire it later if they wish.

For rogue trader level of play give everyone + 5 to all characteristics, and around 3000 xp with some spending limits attached.

Unholy_Ravager said:

Hmm. How about common-craftsmanship Shock Maul or common-craftsmanship Riot Sheild?

 That seems to be a viable idea.

For rogue trader i was thinking regimental/ Ordo bonus could be replaced by ship roles bonus.

Unholy_Ravager said:

Why bother? RT already has the excellent lifepath system.

 Skills would need to to be modified slightly due too differences in the skill list.

 I'm vaguely alright with these rules so far? They need to be tested.

In the mean-time… Only War rules for Tau, anyone?