In my personal experience...
with morgana, i build my item as my team needs, not as i need.
you're on a team with 3 tanks (whom usually have good disables) and maybe 1 other dps
then you're team lacks DPS....so you should focus more into building an AP item build that game to help put the damage out there with soil/ulti and use dark binding for a nice nuke
or, you're on a team with maybe 0-1 disabler, and that can easily spell trouble for any team....so you build to reach -40% cooldown to get as much dark binding snares out there as you can to help you're team out, and maybe start building some AP items along with it but prioritizing some HP/armor and -cooldown (ie....depending on your mastery/runes... personally i have 16% cooldown redux from runes, 3% from offense mastery... so i get a glacial shroud, and thats 39% redux.... or WHEN im specced utility i have 16%+9% = 25... get a soul shroud for +HP/cooldown of 40%)
that's basically the 2 ways i build morgana
Mekai's > chalice of harmony (amazing for Mana regen/magic resist)
Rod of Ages
Upgrade boots to sorcerer's or swiftness
(last item can be anything, more AP, guardian angel for armor/MR...abyssal for AP/MR...whatever you want, the game rarely goes this long anyways)
(rarely will a game reach out this long)
Mekai's > chalice of harmony
Soul shroud *or* Glacial shroud>Frozen heart (or both if you have 0% cooldown from runes/mastery)
Rod of Ages
and from there on...zhonyas/abyssal/survivability ... whatever the game dictates you need more.
those are just my personal playstyles though, to each their own.
For summoner abilities i commonly use ghost/teleport