Considering this deck is only meant to be played against my roommates, this deck doesn't really fall into any category but casual. And this was put together by means of what iv already got. Just looking for a way to maybe make it better without spending a lot.
15 Petitioners is too much in my opinion, because you really only need one. About 2 playsets (=8 copies) should suffice. You can add other advisors instead, which increases your deck's options. Like Trusted Advisor, which can give you a blue spell each turn for Drowned Secrets. Bounce Fblthp, the Lost or something similar to draw extra cards even. Or the Wall of Lost Thoughts for more milling. Minister of Impediments gives you a nice defensive tool by keeping large threats from attacking you while also being an advisor to tap for a massive petition once you have four. Changelings are advisors, too. And having a Shapesharer lets you compete with another player's big threats. You could even go for the changelings' predecessors, the Mistform critters from Onslaught block, like Mistform Wall.
Or simply make every creature of yours into an advisor with Xenograft, Arcane Adaptation, or Standardize. Heck, in that case (well, the first two), you may want to run Undead Alchemist, making advisor tokens for each creature card milled, to be tapped for more mill and more tokens. (Oh, and multiples of the alchemist are cumulative, so you get a token from each one for every creature card milled. Even though only one can exile the card, the others don't care, and will simply do what they can, namely making a token.)
10x Persistent Petitioners
3x Vantress Gargoyle
3x Wall of Lost Thoughts
1x Erayo, Soratami Ascendant
1x Teferi, Mage of Zhalfir
2x Kami of the Crescent Moon
2x Brainstorm
3x Serum Visions
2x Stream of Thought
2x Time Warp
4x Drowned Secrets
3x Visions of Beyond
4x Rhystic Study
20x Island
Edit: I took out Opt for Serum Visions and Folio of Fancies for Stream of Thought.
Or simply make every creature of yours into an advisor with Xenograft, Arcane Adaptation, or Standardize. Heck, in that case (well, the first two), you may want to run Undead Alchemist, making advisor tokens for each creature card milled, to be tapped for more mill and more tokens. (Oh, and multiples of the alchemist are cumulative, so you get a token from each one for every creature card milled. Even though only one can exile the card, the others don't care, and will simply do what they can, namely making a token.)
Visions of Beyond may be a nice card draw spell for the deck.
But really, probably the best way to improve the deck is to add some more lands. 20 isn't enough, even with your cantrips.
Former Rules Advisor
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(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)