Have put a couple of leagues into developing this, most recently picking up a trophy with the UG version here:
I have been experimenting with Mystic Sanctuary since its release, first by just throwing it into Arcanist shells that release weekend on MTGO (wasn't very hard to find on MTGO then), and later building around it more with the RUG Walker list that used Wrenn and Six. A key synergy is Gush and two Mystic Sanctuaries lets you recur the instant or sorcery of choice from your graveyard, whether Time Walk, Ancestral Recall, or something more boring. However, you are limited by the number of lands you can play in a turn.
Fast-forward a bit...In fooling around with Vintage Unleashed, I started with a Gush-Fastbond deck and was incredibly impressed how powerful that engine has become with the addition of Mystic Sanctuary. Sanctuary means you keep Gushing for as long as you have life points and you can recur whatever you want - you don't need to make use of the format's unrestricted Yawgmoth's Wills. I was winning with Hedron Crabs, Lurrus, and infinite Time Walks at that point but the shell is malleable. With the change in the companion rule, I was consider Uro - the life gain from Lurrus was helpful as it made up for the Fastbond life loss and allowed you to go infinite.
And that brings us to the current approach. Growth Spiral was being talked about as in the context of the Arena PTs as both something player's thought was busted (it's the internet and I have no opinion really on formats I don't collect data or play in) and were sick of since it's been in Standard for nearly two years and good the entire time along with Wilderness Reclamation (that one makes more sense to me...). And for better or worse, if cards are great in Standard, they are generally powerful enough to be fringe playable or better in Vintage. That's just the state of power creep in recent sets.
Growth Spiral fulfills the function of being able to play additional lands in the Sanctuary recursion engine (as does Uro for that matter). It does that while pitching to all three Forces in the deck, ramping up to 4 Islands, and cycling. As a card, I was very impressed by it. There are also fun things you can do like Daze back a Sanctuary, Growth Spiral it into play at instant speed to put a Mental Misstep or Misdirection on top of your library, then Gush into it or otherwise draw it at instant speed. The main
consideration though is these additional land drops mean you just get to take a bunch of consecutive turns and win the game. I took 8-9 turns on stream last night during which my Landstill opponent killed my first Brazen Borrower, but I was able to find, play, and attack for lethal with the second Borrower. Both Uros had already been pathed.
By no means is this a finished product. There are other color combinations - I typical start minimalist to identify certain needs and then add colors that I think will help plug in the gaps. For instance, I started with a Bant version for Swords and Mentor, because I thought I would want Swords to deal with threats like Sprite Dragon and Mentor to win the game. After a 3-2 league, Mentor wasn't necessary since you take so many turns that you can literally attack with a 3/1 flier, and Swords was mostly clunkly in my hand without Dack Fayden to filter. I figured a bounce spell was enough to buy time against Sprite Dragon and Brazen Borrower could condense the slot while allowing me to cut White. I didn't get to play against Hatebears or Eldrazi, which would have presented more of a challenge for a deck without Swords.
Oh, and you need a bunch of lands to maximize your explore effects and the odds of having three Islands for Mystic Sanctuary. I cut Black Lotus to help find the slots, but I recognize that not everyone will be interested in this, regardless of the reasons behind it. Just keep in mind that deck design involves balancing competing elements so if you trim lands, that has an associated cost. I actually went from 20 lands to 21 after the first league because I thought it was too few.