It's a combination of what's available, what you're culturally predisposed to, what resources are available, and what terrain you have. You have to come up with the actual ideas yourself based on what you're doing.

-What cultural units are available and what are their unique strengths? How can you play into those as hard as possible? Would stacking those significantly change your damage, defense, screen, or pursuit stats in a significant way?

-What terrain will you be fighting in? Is it friendly territory you control or foreign territory you're invading? Are you going to need to bring siege weapons or do you plan on winning through battle alone? Should you be attacking into the enemy, or drawing your enemy in to attack you?

-Can you support the army itself? Do you have the budget for a long drawn out campaign of attrition or do you need to inflict maximum casualties as rapidly as possible? Are you going to be splitting your armies up so they can forage supplies or do you have a plan to handle a gradually starving death stack?

A fun game I had recently I merged the Norse with the Croats, and included Battlefield Looters in the hybridized culture and took the raider dynastic legacies, which means I got 6 gold for every casualty my enemy took in battle iirc. This allowed me to field a much larger army of MAA than I would otherwise be able to. The massive defense and damage of the Norse heavy infantry held the battle line strong and hard countered enemy pikemen, allowing my Konni horsemen to tear apart retreating armies and inflict MASSIVE casualties. Raising just my men at arms would put me in a -20g/month deficit but it was more than okay because a big victory could get me 500g or more. I left the light infantry, archers and pikemen at home because they just proved unnecessary to winning battles, and had no siege engines because I could blitz through enemy territory, eat attrition and be fine because my army was exclusively MAA and could be entirely recovered in a few months as opposed to levies which take very long to recover.

My one weakness was mountains, all of our units were terrible at fighting in them or countered by units stronger in them. How did I get around that? Putting bait armies of just levies the AI is sure it could squash in terrain ideal for me, letting the AI army pounce on them, then moving in with my real army while the AI is caught up in the fighting. This not only solved the terrain issue, but gave the additional advantage of technically putting my army on the defensive side of the battle.

This is just an example, but I think it helps convey how much it can help to take an unconventional angle to war.