Just to put a proper space with some grounding to the discussion, as there are some merits to be found in taking a look at the secrets for various classes.
Soo, on first glance some secrets are very similar. Mostly Ice Trap, Oh my Yogg and Counterspell.
functionally those are very different though.
first we look at Ice Trap - it returns the spell to hand and enhances the cost. So it does not affect the value of the spell, but the tempo of the play.
Counterspell on the other hand removes the spell form the game, thus affecting value and tempo.
Lastly Oh my Yogg - now in an ideal world the thought process probably is that exchanging one spell for a similar costed one does not affect the value, but in practice it just comes down to pure RNG how it affects tempo or value. The card is really badly designed and should never have been printed.
SO that is just one example where the Mage secret clearly is tuned for value.
And to make it clear, other classes also have quite a lot of secrets that aim to generate or destroy value.
What makes Mage an outlier currently though is that there´s no good option for tempo in the mage secrets.
The common theme in most of the actually used secrets is that they aim for tempo. E.g. Rogue would have tools to generate/destroy value in plagiarize, shenanigans and shadow clone - but almost noone plays those.
And for good reason, Value was taken away as a valid strategy with UiS for the most part. So while Mage secrets currently feel terrible for the most part - it´s not necessarily because they are bad, but mostly because the (heavily tempo focused) meta does not allow for them to be good.
And this leads to a huge conundrum for mage, as for 3 mana making the secrets aim for tempo is very hard, as by definition you spent 3mana for sth. that may not trigger in the next few turns, so you always start with a massive tempo loss and to make up for it the trade off needs to be probably unreasonably huge.
This is much better to balance around at 2 mana where missing the turn is still hurting, but far less than at 3.
So i think while it´s somewhat classic for Mage-Secrets to cost 3 it really limits design space too much and if we don´t want to wait for the meta to one day make value a thing again so mage can have actually playable secrets
My proposal is to just get rid of the class specific mana-restrictions for secrets. They really serve no good purpose. Ofc if we have mixed secret-cost it´s much easier to find out what secret is played.
But in practice you know already which secret is played anyway in the current meta, as there are very few any sane person would put into their deck. So to hell with that arbitrary chains, let´s have some 1 or 2 cost mage secrets for a change.