hey
this is what i can se, is the best team, going to take it part for part, and its also a build for each chap, alot of detales, but hope you se on as much as possible =)

morgana mid, maby karthus are bether, not going to talk abote that, but my opinion is that morgana mid, is werry good
start buy focus on farming, and try to avoid to take damage from the other mid chap, your W if big enouf to damage all the minions, and with your W at lvl 3, you kill range minions, and 4 you kill the meele minions to, with you passive you get some healing, and use clearety when you get low on mana
the runes from morgana is magic pen, and mana, mana for staying in lane, and magic pen for damage dealing

on items, you start with dorans ring, then you shoud be able to stay in the lain to you get 1100g, and maby 2300 (also get the soulstealer)
somone dislike the stacking items, but i like they, ok, not the best late game item, the soulstealer, but earlygame, its nice ap, and if you gank somthing at all, you get stacks, stey passive, and gank when you can

from there you get the best AP items, and by chousing the hourglass and abbysial scepter, you also can take some more damage, also, when you use your ulti, you can use the hourglass, and hold the ulti in proggress, then its only 2 sec left, before you stun, and delt more damage

the best tank, becouse of some resons,
1: the 35% redused damage you get from killer instict
2: when you activate your ulti, you get focused, with 20stacks on leviathan, you take 50% redused damage, then you dont die as fast as with a AP Katarina

you lain together with karthus, using heal, to heal him, before he use ulti to K.O others, you get assist, then you fast get stacks, also you are able to stay in lane
play a bit passive to dont get ganked, but you are werry good to gank, by shampoo to the enemi, slow him, and then use your ulti

again, as on morgana, you buy AP items with armor/magic ressist, and with the weel, you can do some damagewith your ulti, becouse you are imune to the first cc ability

sorry, but havent play karthus that much, just know how to do it on build and abilitys
but K.O the enemies with your ulti, and guard lain when katta gank (she are bether to gank then you, as champ)
remeber to have your E on when you got enemie champs in range
sorry for short part, but are not a good karthus player, at least not yet

play gangplank alot, and like him werry well, him isnt needed so much tips i think, he play kind of suport on this team, using the ulti to help your teammates to catch up with runing enemies, or use your ulti as karthus ulti, if karthus`s ulti are on cooldown

use parrrley to damage the enemie chapms, and attack creps, but you dont need all the minionkils, you get alot of assist on your ulti, and also you are the only on this team with gold grain item, its more important that sion, the one you team with, get the kils he both get health, and he need the g
also you are the one that normaly get the kils in your lain, with your perrrley
use your E when you are taking out towers, and to catch up with chaps

sion, as AD he are carrie
early game you use your shild to be able to stay in line as long as posible, i have got 3k+g first time i am back to shop, so starying in lane is not a problem
but still, you are the best late game chap here, also you are the tank, you get around 3k hp at 40 min (minionkils + base hp), and if you get morgana to shild you, you cant get stuned, then you got (late game) a shild vs magic damage that can take around 1000 magic damage, your shild, that taking 300 damage, 100% lifesteal, criting 1100-1500 (1500 when you have get down the armor with the clevor, witch you normaly dont get), and 2.1 attack speed (try to stey close to ganngplank, then you get 120% lifesteal, crit 50 more, and 2.3 attackspeed)

can tell abote once, (the time i did understand how op it was with AD sion)
was ganked, ashe, ramus, gangplank, twitch, and mundo, did stand there alone, and did get tounted by ramus, when that was out, i did have 300 hp left (did just watching the hp bar, so yes, 300-400), 3 hits on ashe, that was way to close xD, = full hp on me, 0 hp on her, then i was starting attacking gangplank, down on 2 sec.
but at the end of the mundo killing, my lifesteal was out, so i did just kill mundo, before he did run away, and then i did flash ower the wood, and safe i was, with full hp(ye, i did flash with full hp, all was done with build, and you dont stand a chanse vs a ramus with thornmail, and was just abote geting tount ready again, and a good wtich player)

in an teamfight at least late game, your team are werry good with 4x good area damage, and 2x good meele damager`s

karthus`s area damage, then katte use killerinstikt - shampoo - ulti, gangplank blow hes ulti, then the moovment, and attack damage boos, and start attacking, morgana use ulti right away, then use her others abilitys, shilding 1, so he dont get stuned (katta, sion and gangplank is afther my opinion the moust important, karthus AoE damage is what so ever counting.

also he got bonus armor and magic ressist, the same with morgana, ehr AoE damage (ulti) dont stop if stuned, and she dont delt alot more damage, ye, good with some other abilitys, but still.....

but gangplank, katta, and sion is needed that dont get a cc on they
with this team, you also get a nice output of bouth magic and physical damage, gangplank + sion dealing more damage then whole of the other team (normaly they do, and almoust sure they do so if long game, and nice teaming from your side), and with karthus, morgana and kattas magic damage, they can pick a avearage team, only they 3

at earlygame, my tips is that karthus and gangplank let ther sion and katarina get the minionkils, that give they a boost, not an important, but if you want a litle boost, it may be a good team you playing vs, and feeding katarina and sion on minions, is a good idea.
karthus and gangplank get g anway, karthus and ganplank are they that normaly get moust kils where, bout got long range ulti, and bouth are the one that dealing the moust damage to the enemie champ in the lain, morgana going solo, no point in dont taking the minionkils

the sumoner spels are not the part of the build i have been thinking abote, its the speels each of the champs need to get the best posible game, but remeber, dont pick heal on katta if you dont lain with somone that got a long range ulti (karthus, gangplank, and twisted fate can kill the eniemis so fast afther your heal, that you get assist on it)

pros/cons with this team
pros:
- possible the best damage output team it are possible to make
- nice mix of magic and pyhisical damage
- alot of flash and exhost, its almoust imposible to ecsape
- cheap/"easy-to-get" build on katta and morgana
- alot of AoE damage, and AoE healing (sion)
- done with build on all of the chaps = sure on win
- good to push down towers
cons:

-build on sion,gangplank, and karthus, that are so expensive that you get done with them only 1 of 20 games

- got to play passive between 10/15 - 25/30 min, not the best mid game team, ye, alot of damage output then to, but still, you dont got a tank before 30 min (then katta got weel, some stacks on the hp think i dont remember the name of xD, and maby the amor thing that building to the hourglass, also, at that time sion starting to get bether, he are good weery early, with a good stun in lvl 1, but then he are not good before he got some attackspeed from the phantom dansers

if it is somthing i have forgot, or somthing you want to know, just send a message, or write a comment