I believe, as has been pointed out in a number of other threads, that it is more player ability that will determine a winner, rather than the "tier" of the mage used.
I think nobody who has played more than a couple of MW games will disagree with that statement. Especially no one who is still active in this forum. Skill is the most important factor but you cannot use it as an argument for not balancing the mages properly. And they are _not_ balanced right now. Not at all.
All up I would rate action generation as more important than mana generation. If I can do 4 actions on my turn and you can do 2, then in more games than not I will beat you.
I'm afraid that is a too simplistic approach since 4 spawnpoints do not benefit you if you don't have the mana to properly use them. Proving that action-advantage of its own isn't worth all that much. And you can expect that there are books that have a 100% winchance if you start a match with the Siren by casting 4 spawnpoints with your first 4-6 actions
Knabb's suggestion on the other hand seems quite... carefully thought through imho.
He notes that changing mages themselves should probably be the ultima ratio and I agree. Necro certainly is very strong, but as he said, it mainly is because of the brute (+ graveyard). If we'd nerf the brute it would automatically and directly weaken the necro as well because the "zombie-necro" is far stronger than every other necro.
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Before we change anything we should ask ourselves: What is our goal?
Should it not be that in a perfectly balanced game you can play every single mage in a tournament? And even better, shouldn't the balancing be that good, that you can even play those mages in a different way?!
For example if I see a necromancer high up in a tournament, I know exactly what is waiting for me.
I know, many games who have a much higher budget as MW does do not even achieve my second standard because it's so hard to pull of.
Take Magic for example, you can play (constructed and limited) at the top with every color. But often once you see the color, you know more or less what his deck is about and which cards he has in his deck.
How to achieve this 'first balance level' is relatively easy I dare to say. (The one that every mage is playable in a competitive environment)
As long as we have a clear powerlevel difference we can easily adjust the manacost. Either make keycards the current top builds use more expensive or make keycards weaker decks use cheaper. It's not complicated, just a lot of work... because messing with the card-abilities, general concepts or how the cards interact is far more complex and not needed at that point.