Current ruleswise, Guillimam is no longer unique in his support role. A chapter master and Lt. provide the same auras for fewer points. You can even make an Imperial Fist chapter master provide the same buffs in one unit. Guilliman brings presence and the Emperor's Sword to an Ultramarines army and forces your opponent to chose to kill him or the Leviathan/other threat. Russ, Lion, or Khan would do the same.
Russ would make more sense later, narratively. If they didn't bring him back in Wrath of Magnus, they don't need to bring him back to fight Necrons. He needs to come back after Fulgrim and Angron have been released and be the anti-Primarch Primarch with a souped up Wulfen aura (+1 A for all space wolves while he's on the table, +3" charge aura) for his army and he'd also have his psychic howl requiring a caster to have to roll the psychic test plus a D6 value to cast a power. He would be more about being a good model himself, where the Khan and Lion would be more about making their armies better.
Khan would work at anytime and hopefully herald some new Drukhari stuff. He could amplify his librarians and bike units (somehow bring back HQs on bikes for White Scars). For all the talk of Russ having two forms, let Khan be that Primarch, with a T9 jetbike option for more points. He could allow your forces to forward deploy as his gimmick at the cost of not taking any reserves maybe.
Conversely, Lion would provide command benefits ( for whatever command points look like in 9th) and allow you to reduce the cost of dark angel strategems and provide an Astartes Vect action through some Watchers mechanic. He could also bring in a unit of reserves per turn for free if that makes sense for the reserves mechanic at the time.
For the Imperial long shots...Dorn returns and owns the castle mechanic, providing short range buffs to toughness and saves, and allows Imperial/Crimson Fists and Black Templars to all be taken as one army without losing anything. You roll for Vulkan each round to see if he's on the table. He carries a hammer with a 6D6 damage swing and has better buffs for Salamanders but can buff any Imperial army. Corvus can deploy 1" away from enemies from reserves/deep strike instead of 9". Basically Lias gets legendsed and Corvus takes all his rules. Maybe the Ghost of Ferrus allows IH players to play with rules from the ghost of their supplement.
On the other side
Fulgrim requires a 6 to be hit in melee and has a 2+/3++/4+++ save mechanic. He does more damage the longer he's in combat with a unit, like each consecutive turn his melee damage doubles, so not even other primarchs could survive more than two rounds against him. This would force you to commit multiple units solely to Fulgrim's mechanics, thematically making the battle all about him fitting his character.
Angron would just kill. Non-Character Infantry units just die if he gets into melee range of them and he would do one 10 damage hit against a vehicle on the charge. The gimmick would be he would have to declare a charge against everything in an 18" range to get these effects, but that range would be reduced if Kharn is nearby. He'd be able to make his normal attack roles, then roll a D6. On a 3+, he and any world eaters in range would attack again immediately, on a 1 he'd attack again but also attack the World Eaters. Psychic powers would always do max damage against him, but he would regain a wound per unit killed near him.
For the Chaos longshots, Perturabo would give +1A,S, and AP to all IW weapons while he's on the board and provide similar command discounts to the Lion. Lorgar would buff demons and get free summons each round summoning better demon each round up to a greater demon by round 5.; He could mark a number of Word Bearer HQs to automatically become demons princes when killed, and Lorgar would be second to only Magnus on casting buffs.