Champion balancing should make them viable for any team composition.
Ideal champions
Teamfighting, Laning, Damage vs tanks, 1v1 potential, Zoning power. 5/5
If we had these three champions in mind when balancing the game, we would have a much better time with balance.
I always like to see these guys on my team, and when I pick them I feel that they can be played with almost any team comp.
There's also a-lot of different build paths they can take depending on who they are fighting thanks to how item design was made for mages.
Best against squishys for example, 
Typical tank killer item
Let's compare a few champions
3/5 Can't teamfight well, can't do much to tanks/heavy CC
Honestly I would love to have Zed on my team, but more than enough times have I seen him go 5-0 and have no impact on the game.
Essentially a lost at champ select situation.
Now that we know he is weak in these aspects, we can now move on to reworking him to make him better for teamfights + tanks, without overpowering his already strong laning and 1v1 potential.
Now lets take a look at one of the most busted champions right now.
5/5
He essentially excels in every aspect. Teamfighting, Laning, Damage vs tanks, 1v1 potential, Zoning power
Galio can literally run in a straight line and make your whole team run away, even tanks. Who doesn't want to be hit by his whole combo?
At this point from a balance perspective it would make sense to weaken him in one of the 5 traits mentioned above.
Targetting his 1v1 potential, while keeping him as a good teamfight champion. Simply reduce his taunt and knock up radius.
But if we blindly nerf his laning phase (His tornado) we actually nerf him entirely in all 5 zones, which would be bad.
Let's look at one of the weakest champions right now
He used to be good at everything, but then we saw targetted nerfs to his ultimate.. then his Q.. then his pillar.
His teamfighting power was amazing thanks to his Pillar cooldown, and his Q in combination with his ultimate was the best tank killing combination ever.
So at this point a simple fix would be to buff up his Q and ultimate up again, except this time we keep in mind how good he is at damaging tanks. RATHER THAN his 1v1 potential against other champions.
Q no longer reduces damage from target, trundle still gains AD. Base damage increased.
Piller cooldown reduced, slow removed.
Ultimate steals a flat amount of MR/AR from the target, rather than a percentage.
Trundle now maintains his champion design concept of "Zone control" We could even go into detail, and make his pillars SLOW enemies if it is cast inside of his W.
Anyways let's keep this thread interesting. Here's a simple template I would like people to use in the comments below. Champion 5/5 or 3/5 whatever it is. Explanation of the rating. Implemented changes without weakening them in multiple aspects.