It appears that we now have the details for the “Victrix Guard” formation, which is an Ultramarine only choice that can be chosen as an Auxiliary for the new Victrix Strike Force (UM Decurion equivalent) or taken as a “stand-alone” formation.
The Victrix Guard consists of:
-Captain Cato Sicarius
-1 x Honor Guard
4 x Sternguard/Vanguard Veterans (any combination)
***Important Note: None of these units can take Dedicated Transports or start the game in a transport, which has important implications that I will discuss below.
In terms of special rules, they get two quite good ones:
 All units in this formation gain +1WS/BS for free
 Guilliman can use Look Out Sir! with any units from this formation, despite being a Monstrous Creature, as long as they are within 3″ of him.
So, how does all this stack up, and how can it be applied to competitive play? Well, just out the gate, WS5/BS5 on Honor Guard, Sternguard, and Vanguard take them all up a notch (Sicarius gets WS7 and BS6 as well, but this is not really that big a buff for him over what he already has), from “very good” to the “outstanding,” in my opinion, especially when combined with the different Doctrines Ultramarines get access to. Weighed against this, of course, is the very big negative that they cannot take Dedicated Transports (so they have to hitchhike on Fast Attack Rhinos or Heavy Support Land Raiders from an allied UM CAD) and they cannot start the game in a transport (so no Drop Pod riding at all, and basically no chance of riding in a Stormraven, unless it starts on a Skyshield Landing Pad and they hop in after the game begins). Finally, on initial impressions it appears that Captain Sicarius is a 37 MB “tax” to use this formation, since while he has good abilities, he is not widely considered a competitive choice for an Ultramarine army like Tigurius or Calgar is.
After digesting these pros and cons of the Victrix Guard, my first take was this it was probably not worth taking, despite the great buffs to WS/BS, because of what a disadvantage it is to not be able to take transports (i.e. Sternguard especially depend on Drop Pods or Rhinos to be used competitively). However, as I have thought more about it, I began to see some ways that you might be able to use this formation and still field a half-way competitive army.
Here are some ideas I came up with, in that vein:
 Footslogging Guilliman Bodyguard — This may be the most obvious option, and the one GW intended us to use given the very specific special rules for LoS! they included in the formation. Take 4 squads of Vanguard Vets (3 with Jump Packs and a mixture of close combat weapons to taste, one foot-slogging with maxed out Storm Shields), plus the mandatory Honor Guard and Sicarius. The three Jump Pack Vanguard squads operate independently, either coming in from DS Reserve (Sicarius’ +1 to Reserve Rolls helps here) or advancing up the board rapidly on turn 1 to get into assault as soon as possible with their WS5 attacks (with re-rolls to Hit of 1, thanks to Assault and Tactical doctrines which, with Guilliman on the board, you should be able to activate a total of four turns a game). Meanwhile, Guilliman footslogs up the middle with the Stormshield Vanguard, Honor Guard, and Sicarius nearby…. if you gets lots of enemy attacks the Guilliman can tank easily (thanks to his T6, EW, 2+/3++, FNP, and 6W), then he just shrugs them off and the whole battle group advances as fast as they can come to grips with the enemy.
If, on the other hand, he gets hit with lots of firepower that can kill him efficiently (Grav, D-Weapons, massed poison, etc.), then use the Look out Sir! to pass those wounds off on a 4+ to either the Stormshield Vanguard (for the AP2/1 attacks, like Centurion Grav Cannons) or to the Honor Guard/Sicarius (for the massed AP3 and worse attacks like Scatter Laser Jetbikes, DE Splinter Weapons, massed Tau Missile Pods or Smart Missile Systems, etc.)…. remember, the way Look Out Sir! works is that the to Hit and to Wound rolls go against Guilliman FIRST, then he can decide to try and Look Out Sir! before saving throws are made… this is important because it means that all the attacks must successfully wound against his T6 FIRST, and then the nearby Vanguard/Honor Guard can still attempt to absorb the attack with their 3++ and 2+ saves, respectively (i.e. put another way, you get the advantages of Guilliman’s high T6, but still get to use their saves and wounds, so kind of the best of both worlds). The entire point of this strategy is to keep Guilliman alive as long as possible so that he can keep buffing the rest of your army and also get into combat himself, where he will pretty much wreck havoc on anything opposing him. In doing this, the Jump Pack Vanguard can serve as a type of “holding force” to tie up key enemy units in combat long enough for Guilliman (and his bodyguards, who are solid in close combat themselves) to run up and charge the unit himself.
With this strategy, with whatever points you have left over, I think it would be wise to build as much of a backfield gun-line as you can (i.e. Sicaran Predators, TFC and Quad Mortar batteries, Devastators, Land Speeders with TMLs, Whirlwinds, allied artillery, etc.), both to support you advancing melee elements and to take full advantage of the attention that your opponent will likely pay to the melee units vice the shooting units. If you do manage to get Guilliman safely into enemy lines, especially against a shooty army like Tau or Astra Militarum, then his exceptional melee abilities might very well turn the tide of the game in your favor.
 Gun-line Sternguard — Another way to run this formation does not depend on Guilliman (or the Look Out Sir!) abilities at all. Instead, you take:
-A fortification like an Aegis Line, Bastion, Firestorm Redoubt, etc.
-Four minimum sized Sternguard squads
-Mandatory Sicarius/Honor Guard.
The Sternguard each take two Heavy Weapons like Lascannons, Missile Launchers, or Grav Cannons, and operate basically like expensive, elite Devastators. Place them in the fortifications for improved survivability and then use their BS5 (with re-rolls to Hit of 1 from Tactical and Devastator doctrines) to have a 97% chance to hit with their Heavy Weapons and Special Issue Ammunition bolters (Note: Even if you use up the Doctrines, if you take the Ammunition Dump upgrade to the building, then they can re-roll 1s to Hit all game long).
As for the Vanguard and Sicarius, they form the “counter-assault” element for this Gun-line, waiting behind protection until enemy units try and get close to dislodge the Sternguard and/or seize objectives, then storm forward and tear them up with their 2+ saves and massed PW attacks. Sicarius also uses his various buffs (i.e. +1 to Reserve rolls, Ld10 to the entire army if he is the Warlord, Infilitrate/Scout/ Tank Hunters/Counterattack to a single Tactical squad, etc.) to assist your army while perhaps manning a Quad Gun (with BS6) for anti-flyer/skimmer firepower for insurance on that front as well. Combine all this with some additional shooting, say from Astra Militarum or Cult Mechanicus allies, and I think you have the making of a solid army.
 Issodon Infiltration/Mechanized Force — This may be my favorite, and the one most likely to be competitive in a tournament scenario. First, take the following:
-Lias Issodon (Warlord) along with a Raptors CAD (including at least one Spartan Assault Tank and one Grav Centurion squad, plus a pair of melee Scouts in Land Speeder Storms for the mandatory Troops slot)
-2 x Sternguard squads with good mixture of Combi/Heavy weapons
-2 x Vanguard squads with Jump Packs and good mix of Special Melee Weapons/Storm Shields
-Sicarius and Honor Guard (with Banner of the Emperor Ascendant)
Now, use Issodon’s Warlord trait to infiltrate the two Sternguard Squads, the Grav Centurions, and himself into position before the game begins (Lias goes with the Centurions to give them Shrouded and at least a modicum of close combat ability). Turn 1, the Honor Guard/Sicarius load into the Spartan and then Move/Flat Out 18″ forward while the Jump Pack Vanguard move up behind the Spartan, using it for cover/LOS blocking.
Provided you don’t get Seized on and have first turn, you can unleash two Sternguard squads worth of BS5 (again re-rolling 1s to Hit, thanks to Doctrines) firepower, plus a full squad of Centurion Grav Cannon shooting, on your opponent for a devastating “alpha strike” (without having to depend on Drop Pods, which is a big deal against Interceptor heavy Tau forces, for example). Additionally, you have multiple elite melee units in position for a Turn 2-3 charge, where they can put their superior weaponry and WS5 to excellent use, all the while being supported by the firepower from Spartan and the Raptor Scouts/LS Storms using their mobility to grab key Objectives. If you do not have First Turn, or for some reason do not think it makes tactical send to Infilitrate, then you can have the Sternguard, Grav Centurions, and Issodon Outflank instead (remember, as long as Sicarius is alive/on the battlefield and Issodon is simply alive, you get a re-rollable 2+ for any Reserve rolls, plus your opponent gets -1 to his Reserve rolls). Finally, if you want to start the Vanguard Vets in Deep Strike Reserve, not only can you get them in on Turn 2 with 97% reliability, thanks to the Reserve modifiers mentioned above, but they can use Issodon’s Locator Beacon to DS in with no scatter, making their arrival that much more reliable and accurate.
Overall, while I think the Victrix Guard has some very real (and constraining) limitations, it also has some very great possibilities, as mentioned above. and I think it should be an exciting formation to experiment with and, hopefully, play with at a competitive level.