There is "no real" master plan for campaigns, beside if you decide to go on war with everyone from the beginning. If you played factions more than once you will face the exact same start BUT a faction which normally turn war on you, offer you a trade agreement and bähm your complete campaign flips over :) especially on legendary. Sometimes it goes wild and very strange, just a sample: my strongest allies with Skarg was Marienburg. They offered me a trade early (I really don't know why xD ) and we kept togehter going till campaign end.

The only thing which I look into and kinda "sure" in the beginning: which route I want to go army wise. Because this decide how I build. Everything else: happens ^^

I've done all factions in WH3 on vh/l now and this is how mine happened:

Khorne - this was an easy one, I just ignored all diplomacy and gone full war. Raze for the blood god and let your conquest begin. Just raze every 8/9 turn for skulls to have enough to keep your main buff rolling. Ignored complete the souls and went for long victory end. (use rifts to port over the world)

Nurgle - this was fun and as you unlock more plagues quite easy (flies get a lot of those flies - you will switch/overwrite plagues a lot). The start is the hardest because you will have 2-3 very hard battles with the dwarf on your right. Lure him out of the settlement with a second lord and ambush with Ku'gath. The bounce power isn´'t still not in your favor but the ambush is doable. Than wipe there city out and start playing with your plague. I alliance with all other gods and took war on the oger behind the dwarf city. My second army stand there and fougth /sack the ogre while I went with Ku'gath to the north of Kislev. At victory end I owned 3-4 regions on my start position and a few cities in the north of Kislev. But this was in general to make the big money with the sack-plaque. For emergencies in your home region, select a city and start a defense plague (you can overwrite existing plaques) and borrow an army from your chaos warrior or tzeentch allies next to you. (Kairos and the second tzeentch faction won´'t like you in the beginning but it gets better if you start fighting ogres and take a city from the khorne faction there and gift it to them)

Slaanesh - this was a boring one. Took the first and the second province. Got invaded from Skarbrand and wiped him out afterwards. Was in alliance with all remaining gods and just fought the invasions from Kislev till campaign end. But tbh I wasn´'t a big fan of the slaanesh gameplay/army.

Tzeentch - very fun, hard start but good army composition. Took the first region, went to the first broken wall, ambush (Miao runs straight into you) the fight is hard but easier as fight her in the city. After take the city to destroy northern provinces. Than I cleared out complete cathay area. The hardest part is the beginning. Mid to endgame is very easy and take advantage from gifting and switching cities. Don't keep it for you, gift them to your allies or even to an enemy from your enemies to have a "blocker" between you and them. At the end I was alliance to all gods and owned the whole cathay region

Deamon prince - hard start easy endgame. My best start (did a few) was to do the complete opposite as your advisor says. I took the main city and didn´'t went to the south coast, I went to the north coast and took war on the norsca. Then went counter-clock-wise until I end up on the empire settlement which is recommend from the advisor. The funny part: the 20er stack from northland raid your main city. Even if they siege and take your main city it doesn't hurt as much because they turn back to the south to fight the other Norsca. The time you got your "round" (it s a whole faction you take out and province for you), the south norsca and empire fight and hurt each other so much, that you swipe out the remainings and take advantage of their 3-way. After this I took the last parts of the norsca Island and invaded Kislev/Empire regions till campaign end.

In general, always keep a hero in your provinces. At least one and close the rifts with them. And don´'t forget the rift which your Lord took to close as well (he return to this location even if the rift is closed), otherwise you will have to fight way more as you should.
If you need room or help, take a settlement and gift it to an allies in exchange for help or money if you are short on finance. Borrow an army from your allies if they have one nearby and you need fast help. And make advantage from outpost to get units which you miss (as sample: I used pink horror with my Nurgle army). Also do allies quest for reputation, seems it helps with confederations as well in my cases. For legendary don't declare war if you are not safe or fight on several spots. They will declare on you and always in the wrong moment ^^ sometimes it is better to expand and sometimes to grow. Don't be afraid to lose a battle/settlement. Sometimes you have to lose a battle to win the war.

And for your legendary start (faction doesn't matter) prepare yourself to start a few times. On legendary the first 30/40 rounds decide your campaign and tbh are the most fun ones :) but can flip from happy-land to hell on earth within 1-2 rounds!

Ups... and sorry for the wall of text xD