Well, for one, convoke/improvise style cards, or anything with additional/alternate costs, get weird rules-wise if you generally pay costs before casting. Since now you’d be doing part of the cost up front, and part while casting (since you can’t do them before casting, they’re usually illegal game operations without the card instructing it happens). It wouldn’t be a deal-beaker, but it would probably make alternative costs less intuitive than they currently are. Plus, I suspect new players would have to get constantly corrected about playing cards before tapping their mana, which feels pedantic and doesn’t add much except annoyance.
And if you mean making mana generation instant speed, with priority timing, that’s a whole other can of worms, with terrifying implications.
Well, as one mechanical concern, you can't use pharika's ability on your opponents' graveyards to generate snakes for yourself. When you exile a creature card, the creature card's *owner* gets the snake, not you. So only self-mill and exiling your own creatures gets you snakes to pillow fort behind. As your deck only has 9 creatures total, that's not a lot of snakes total, which make your sac engines, etc. kind of weak.
Plus, your deck is *very* artifact and instant/sorcery heavy, while being light on colored permanents. So, that means activating pharika's devotion will be very hard. If she's just an enchantment, and not a creature, she's missing her best features.
Mill may be kind of tricky in these colors in general. Most of the best mill cards live in blue or dimir. Not saying it's impossible, but it will have to lean heavily on artifacts... which is a non-bo with pharika, and since she wants *self* mill to have snake exile targets, she has very specific mill needs. (Or general "lots of her own creatures dying in combat" needs, which doesn't work well with a low creature count)
There's plenty of specific card choices to argue about one way or another (some of your mill package is *much* better than other parts of the package, and I'm not a fan of fyndhorn pollen in a deck where one of the central plans is "have lots of 1/1 deathtouchers ... that are now 0/1 deathouchers"). But, I'd work through about the big picture before worrying on the details here.
Well, just to mention, if she doesn't want a stock merfolk list... what about a *non* stock merfolk list? There are a *lot* of quality merfolk cards out there that are probably good enough for modern, which just don't make the 'stock' lists by virtue of being a little less consistent or being bad meta calls. Coralhelm Commander, Cosi's Trickster, Merfolk Sovereign, Thassa, God of the Sea... even more corner-case cards like Lullmage Mentor or Dakra Mystic aren't unplayable. Hey, throw a bit of red into the manabase and Jori En, Ruin Diver is playable too. As long as she's running enough lord effects it'll run away with some games in classic merfolk style, while maybe matching her aethetics/playstyle better (more combo-ish? Aggro-ish? concerned about art/theming?).
And if you mean making mana generation instant speed, with priority timing, that’s a whole other can of worms, with terrifying implications.
Plus, your deck is *very* artifact and instant/sorcery heavy, while being light on colored permanents. So, that means activating pharika's devotion will be very hard. If she's just an enchantment, and not a creature, she's missing her best features.
Mill may be kind of tricky in these colors in general. Most of the best mill cards live in blue or dimir. Not saying it's impossible, but it will have to lean heavily on artifacts... which is a non-bo with pharika, and since she wants *self* mill to have snake exile targets, she has very specific mill needs. (Or general "lots of her own creatures dying in combat" needs, which doesn't work well with a low creature count)
There's plenty of specific card choices to argue about one way or another (some of your mill package is *much* better than other parts of the package, and I'm not a fan of fyndhorn pollen in a deck where one of the central plans is "have lots of 1/1 deathtouchers ... that are now 0/1 deathouchers"). But, I'd work through about the big picture before worrying on the details here.