Zilean is the only one I sort of agree with, in that being able to have a free GA if the mana costs were to be standardized it might also require adjusting numbers on his ult. That being said, i feel the rest can be set to have the standard 100 at all ranks without it drastically changing the balance of the game, because in most cases there are already other champions who can do what each of these can already.
Amumu - Locks down enemy team for 2 seconds. This is comparable to 2 other champions, though each do things slightly different. Galio has a 2 second lockdown with a range 50 higher than Amumu. While Galio doesn't stop champions from attacking, they are forced to focus him at reduced damage, while his ult gets stronger for each hit he takes. Nunu also has a large zoning ult, though instead of a complete lockdown champions have significantly reduced movement and attack speed. In exchange for this not being a perfect lockdown, Nunu has excellent base damage and AP scaling, provided he can get his full channel.
Orianna - While there really aren't comparable abilities (the closest I could think of was Diana's E, which does no damage) I would argue that her ult alone doesn't do nearly as much when not combined with her other abilities. Alone it has 70% ap scaling, and ok base damages, it only becomes truly terrifying when you can R-W-Q-E an entire team... For this, I kinda see your point as the 50 mana difference at rank 3 basically allows for an extra shot at repositioning the ball. However, I still am of the opinion that this slight buff isn't going to be completely game breaking.
Karthus - while no champions have comparable ults, as I've explained in other posts this is actually the most irrelevant champion on the list. Karthus generally tends to build fairly deep into mana, often having RoA, Tear/Archangels/Seraph's, or both. Furthermore, with his passive giving him free spell casts after death, come late game teamfights a good Karthus player will wait until they die to ult, thus making mana costs irrelevant. If they don't die, chances are they won't need to ult. This could potentially be a buff to jungle Karthus, but still insignificant due to his passive.
Sona - large AoE stun. I would compare Sona in this case to Annie. While Annie's tibbers doesn't always stun, generally when used from the support role the player will have the passive stun up. Annie doesn't have as great a range on the stun as Sona, however it is instantaneous, and scales to a slightly higher duration than Sona's lasts. Furthermore, Annie can deal far more damage with her ult than Sona can, and is left with an extra 100 mana, which results in additional spell casts that have very good ratios and base damage.
Twisted Fate - similar to Nocturne and Pantheon, TF is still more in a class of his own. I would argue that standardizing TF's ult would only be a slight early game buff, as his rank 3 ult is 100 mana anyways. By the time TF can port into a lane and 1v1 an opponent trying to split push, his ult already costs 100 mana. TF has great mana sustain, and an extra 50 mana at rank 1 and 25 mana at rank 2 would be irrelevant in competitive play. It would help players in low levels like Bronze, but this slight advantage would still be gone come late game.
Veigar - deletes mages, still not as good as Syndra. Ult aside, all Syndra's abilities are AoE. Her Q is on par with Veigar's until rank 5 where her passive makes it more powerful. Her W-E combo doesn't stun as long as Veigar's, but deals more damage (as veigar's stun has no damage) and a skilled Syndra will usually have around 6 balls when ulting. Not needing to scale off of enemy AP, Syndra consistently deals ridiculous base damage, 1080 at rank 3 for 6 balls. At rank 1, 6 spheres has more base damage than Veigar's rank 3 ult, with the same 120% AP scaling. Most importantly, she out ranges all of Veigar's abilities.
Lastly, remember Urf mode? All ults benefit from being able to be used more with reduced CD. I'm not talking about reducing CD, but making ults more standard in costs.
