BY Mystic Souls

TL:DR - Play it like any aggressive BY deck, mulligan for early collectors like guardian or BT/water elly, but towship takes priority if possible. Otherwise any of the above can work too.

This is my B/Y homebrew, it plays like any other BY except it's a bit more aggressive. Here's a few guidelines:

Basically try to control the wells, you'll want to play as aggressively as possible, since that's what Towship + Mystic beast is in the deck for. Red is a really bad match-up, but is manageable if you respect firestorm/garudan. For mulligan you'll want towship, BT, water elly, and/or guardian (guardian over Battle Toads, since it's harder to remove), but towship is a priority if you want an early aggressive lake. As a side note, sunken tower + towship is acceptable too.

I'm still not sure about the guardians, i added them because i've had a lot of success with them in other dekcs. They're very hard to remove for most decks, and frogtossers/stalkers can't get any value on them. But the deck is lacking some removal, so Last nightmare/choking sands, or crystal flower might be better.

Khalim's is a great way to add a bit of mobility, especially on Towships. It's also a good way to get that extra oomph to clear 3-6 health creatures, or prevent a favourable frogtosser trade on your fanatics/elementals. Same goes for sunken tower; which has a lot of applications and is a very versatile card (Feel free to add shifting tides instead if you prefer).

I chose Fanatics over flash wind for the extra body for Istanu, also it's a great collector and can clear a lot of cards with a khalim's buff. And finally Twinsoul Spirits is a secondary win condition and works as a 2 for 1 body for Istanu, and is one of my favorite double colored cards in the game. It's great fun to play with, and it has a lot of uses. I chose it over Flight of the mantas because i honestly don't like that card very much.

You might be wondering why i'm not using manta rider? Well it can sometimes force you to make too many early deserts and this is a more blue oriented deck. But it could possibly work, I'm not a fan of manta rider in generel though. (Too cookie cutter for me :P), but if you like it, then go for it.