Unless you've seen the whole set (which I haven't), there are a lot of assumptions here.
Well, I did state in my post that I was making guesses and assumptions since we haven't seen the vast majority of the new cards.
Before PVS there are less than ten water spells. After PVS there are approximately 40. I might be off a little, but I did just count them the other day. This set makes the Water school huge.
I'm not saying the Water school won't get a massive buff from the expansion, but you're looking at an almost non-existent school at the moment. Let's go ahead and estimate that after the expansion the school has 40 cards. Then, guessing completely blindly (because I have no idea how many song and pirate cards are included in the set), let's also add in 12 non-Water pirate cards and 12 non-Water song cards (which is probably being generous). That gives the Siren a total (in-school) card pool of roughly 64 cards. That sounds pretty good, but let's compare that to other mages. The Priest/Priestess has a card pool of 39 Holy cards, based on the card database from this website. But, that doesn't account for the new cards released in the Priestess Academy expansion (roughly 24, by my count). It also doesn't account for the new Holy cards which the PvS expansion itself will bring. Let's low-ball that and say 15 (6 Holy cards have already been spoiled). That gives a rough total of 78 cards. I'll go ahead and list some other Mages as well (ones which shouldn't be as affected by the release of PvS.
Beastmaster = 69 Nature + 21 Academy (roughly) = 90
Warlock = 79 Dark + 18 Fire = 97
Necromancer = 79 Dark = 79
Forcemaster = 37 Mind = 37
Warlord = 17 Earth + 70 War = 87
(Errata'd) Wizard = 22 Air + 47 Arcane + 24 Academy (roughly) = 93
My point is that even with the massive influx of Water school cards, I fear Arcane Wonders is going to have to end up doing what they did with the Warlord: moving a secondary expansion forward so that the card pool is even somewhat on par with other schools. The only mage with a more limited set of in-school cards is the Forcemaster, who also happens to belong to the only school which has not yet received any major expansions after its introduction in FvW. This is the problem with waiting this long to really establish the Water school: most of the other schools are so far ahead that one expansion just can't cram enough cards in to equalize the balance. Despite everything I just said, I'm not nearly as worried about her initial card variety as I am about how that card pool will be expanded down the line (I'll go into detail below).
And just like every other school, there's no reason Arcane Wonders can't include new Water spells (or Pirate or Song, for that matter) in future expansions. There is almost always a new good card for every existing mage in every expansion thereafter.
And some day we might see Frost damage type become a thing, which would also presumably fall within the Water school... so in general, i think the Siren will have plenty of spells she's trained in. But I think something you're missing is that NOT being trained in a school doesn't make it outlawed; All Mind spells cost the standard double, which means all the level 1 Mind spells cost 2 sbp's, etc... so... fill your boots and include as many Psychic spells as you want!
Let's get Frost out of the way first. The community knows of the next several upcoming Mage archetypes. They will most likely be the Barbarian and Sorcerer first, followed by possibly a Shaman and some sort of Vampire. Unless something changes drastically, the next alt pair of mages will be Forcemaster vs Wizard. The next Battlegrounds expansion will be the Archmage. None of those mages are very likely to be the long-awaited Frost mage or have any water training whatsoever (with the possible exception of the Vampire). So even in the best-case scenario, the Siren will be waiting at least a good 2 years before another water mage is added.
With that out of the way, sure, Arcane Wonders can add new cards for the Siren here and there in other expansions. But the bulk of those expansions will always be dedicated towards helping out the new mages and the schools they belong to. The introduction of the Necromancer helped out the Warlock. The Druid helped out the Beastmaster. That's why I really wish the Siren was trained in Psychic spells. She could have leeched off of the upcoming Alt FvW set coming out eventually. Instead, she shares practically no training overlap with any other existing or upcoming mage, meaning her card pool will almost inevitably become stale while other mages continue to acquire more new cards. The Siren is the exact opposite of the original Wizard.
You're right, not being trained in a school doesn't make it outlawed. But it does drastically decrease efficiency with spellbook points (halves them, to be precise). Additionally, most mages don't care very much about the school which they have to pay triple for (with the possible exception of the Warlord). Warlocks don't need or particularly want most Holy spells because they don't synergize with their pre-existing strategies. Same with the Druid: she rarely needs or wants fire or war spells (especially high-level ones). I would actually prefer a Siren fully trained in Water and Mind with two triple-cost schools (Earth and War maybe?) (obviously she would continue to be trained in songs as well). I don't expect anything to change at this point, but I wanted to share my concerns nonetheless.
Lastly, it's too early to be worried about her stats. I'd be shocked if she didn't have cards that boost them in some way. ...I wouldn't be surprised if there is an Aquatic Terrain card, perhaps a "coral reef," that the Siren needs to protect that gives her +1 channeling and +X life, which automatically combos nicely with her built-in Regenerate and other abilities, and which is analogous to Treebond without using up a mage ability. Although maybe that would be too similar to the Druid (?).
Also, Shallow Sea needs to be considered when looking at the Siren. She can run six of them and that gives her a large area where it's harder to hit her, she regens, and she swings harder if she wants.
I doubt the devs would give the Siren something so close to another mage's ability, but who knows.
The problem with Shallow Sea is that, (while a good card and reusable due to Cantrip) it costs 5 mana to use and only covers one space. As I already brought up, mana is already kind of an issue. How is the Siren supposed to have mana to use her abilities somewhat frequently, cover decent areas of the map with terrain so that she and her creatures can play to their aquatic strengths, and still have mana to actually summon and support a decent army of creatures? Maybe I'll change my mind after seeing her spawnpoint or something else, but right now my feeling is that she needs a lot of set-up time and doesn't have the survivability to withstand an early rush, while other defensive mages can beat her through economy.
