Magosi the Waterveil is a tricky card to use. I am not sure I would ever use it without building a deck specifically around it since you have to sacrifice an entire turn just to gain an extra turn.
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Try to fit a Felidar Sovereign in to do something with all the life gain.
You have some good cards in your stock. When building a deck try to come up with a theme or a single card that you want to build the deck around. If you jump around too much then the cards you want might not come up while you get the cards that aren't useful at the time.
I am not sure if Open the Vaults is the real focus on the deck. You seem to jump back and forth between the Discard/Mill and the big creatures. Seems more like the Vaults is a back-up plan.
Regarding the Hexmage, even if you don't use them to kill a ton of tokens its a good early game defense as it has first strike. I saves you from having to trade your creature for theirs. And later game anyone with tokens has to address it or else their Transcendent Master/Planeswalker/Kargan Dragonlord or whatever else is at its mercy.