aa polymorph deck for the modern format
Your deck's biggest weakness is that if your target for Polymorph disappears, you get set back a few turns.
You need a lot more cheap counterspells. Mana Leak and Deprive are too expensive for what you want. You should be Playing Spell Pierce, Dispel, and Spell Snare instead. And if you need a 2-mana counterspell, go with Remand instead.
As far as your Polymorph targets, there are a lot of good options in White, as it turns out. Raise the Alarm is cheap, and Timely Reinforcements against aggro decks is not suspicious of Polymorph shenanigans. You also get Elspeth instead of Garruk in White. Also, Inkmoth Nexus, Blinkmoth Nexus, and Mutavault are good lands to play as targets for Polymorph, and Wind Zendikon just seems silly.
For draw spells, See Beyond should be replaced by Serum Visions. Visions lets you look at more cards than See Beyond, which is what you want anyway. Also, you should replace Jace Beleren with Compulsive Research, because Research lets you see more cards than Jace will, by the time you want to combo out.
With your lands, having lands like Halimar Depths that always come into play tapped is a huge drawback. You want to be playing Misty Rainforest to find your Breeding Pools.
I'm not 100% sure how to help with sideboard, but you will definitely want cards to hate out Affinity, Storm, Splinter Twin, Melira Pod,