Hello!
Our playgroup has been brewing a lot and we've figured out a few general rules of thumb that seem to be helpful no matter what deck you're building.
-You don't want too many high cost cards, you want to have a decent spread of 1-5 cost cards. That's pretty standard for a lot of CCGs but good to keep in mind.
-Pretty much every deck wants some number of fliers. They have a support value of 30, so they're always good supports regardless of whether you're on attack or defense.
-Different MCs are more or less important to their deck, but you always want a good number of your MC in your deck to guarantee you get to promote them and also so you have crit/evade cards for them. The MC is the one card you know you'll always have in play, so you want to take advantage of that. I usually like 8-13 copies of an MC. Not too many more than that though because you don't want to fail supports constantly
-By the same token, your MC is already in play at the start of the game, so you don't really need 1 cost versions of them most of the time. If there are enough promoted versions of your MC in the game, just use those, not drawing a promotion of your MC promptly is a big way that variance can make you lose (sometimes a character only has 2 cards though so you have to use multiples of their 1 cost)
-If they're available, have 3/2 cost versions of your MC in addition to their "endgame" versions, so you can get your MC powered up early to defend themselves.
-Similar to fliers, usually having a healer character or two with the ability to retrieve cards from your retreat area is always helpful.
-Be careful how many sword units you have in your deck! They tend to have promotion forms with awful support values, and base forms that are only good on attack.
I hope this is helpful! As far as the specific characters, were their individual cards of them you were looking to build around? there are lots of lucinas and corrins and tikis so there are different ways you could tackle building decks around them.