So, after sleeping on it a bit and thinking about it, here are some ideas I have on how to best employ Guilliman with other Imperium forces (especially regarding how his 6 Command Trait WLs can buff units within 12″):
 Mass Contemptor Dreads — Since Guilliman provides re-rolls of 1s to Hit for BOTH shooting and close combat, I figured the most efficient units to buff with him are ones that can do well in both of these phases. Thinking of this, Contemptor Dreadnoughts jumped out at me as a unit that might do very well in conjunction with the UM Primarch, since they really are a balanced unit that has about the same mobility as him (6″ movement and Fleet) and solid abilities in both Shooting and Assault. Not only doe they come with WS5/BS5 stock, but they have 5 x S10 AP2 attacks on the charge, all at full I4, and they can be upgraded to have 6 x S6 AP4 Rending shots a turn each. Combined with re-rolls of 1s to Hit and they become hyper accurate (97% chance of hitting) for shooting and, against most opponents, very likely to hit in close combat (3+s to hit, re-rolling 1s). Add to all this their Av13/12/10 armor and 5++ Invul save and they make for a tough target which will force your opponent to choose between engaging them or concentrating on Guilliman (this becomes even better with target saturation, if you have say 4 or more Contemptors running alongside the Primarch). Finally, with Guilliman providing them Move Through Cover and +1″ to Run/Charge distances, they become even more agile and should be able to get across the battlefield quicker than your opponent might expect. The one downside to Contemptors is their high points cost (37 MBs a piece, with the Kheres Assault Cannon), but if want to take a cheaper version of this, just run Venerable Dreads with Assault Cannons instead… not quite as durable or capable, but only costing 27 MBs per model to mass alongside Guilliman.
 Dark Angel Ravenwing Support Squadron — I have already talked about this in previous posts, but this is basically a squadron of RW Landspeeders and either a Darkshroud or a Landspeeder Vengeance combined into a single unit. The squadron then gets some very good special rules, including Interceptor and Strafing Run (so BS5 against basically any non Flyer/FMC targets). Combined this with re-rolls to Hit of 1s from a nearby Guilliman and either lots of Bolters, MMs, Assault Cannons, and/or CMLs and the squadron has a 97% chance of hitting with its shooting at ground targets that can be deployed against either enemy units arriving from Reserve or just normally as fire support for your main army.
 Skitarii and Secutarii Infantry — Whether it is Vanguard, Rangers, Infiltrators, or even the Forgeworld Peltasts, all of these units can use Doctrina Imperatives to boost their Ballistic Skill to BS5 or higher for at least turns a game. Combined with Guilliman and they, just like the units above, become hyper-accurate and can do even more damage than usual with their already strong shooting (this is especially great with Plasma Cavaliers, since they can re-roll Gets Hot! rolls of 1 and with Arc Rifles, making them almost auto-strip HPs from any vehicle in range, be it tanks, walkers, skimmers, Super-Heavies, or Imperial Knights. Secutarii Peltasts in particular, with their multiple ammunition types, can be very deadly when bumped up to BS5 and re-rolling 1s to hit, whether shooting at Hordes with their Flechette Burster rounds or tougher infantry/vehicles with their Kinetic Hammershot ammo. Finally, if any of these Skitarii/Secutarii troops do end up charging after shooting (thinking of Infiltrators here, especially), then Guilliman’s buffs to Hit, to Charge range, and for Move Through Cover all help them significantly.
I am sure there are many other types of units/battle groups that will synergize well with Roboute Guilliman, but these are just a few that jump out to me right away… overall, I am quite excited to see how he will play out on the competitive tabletop (and we haven’t even looked at how he buffs Ultramarines yet!).