I like the idea of having some useful spirit in the main, but when i look up for interesting options, I end up a bit frustrated, because kataki, labyrinth, eidolon and some other are really specific and I'm not going as far as having them MB.
With this said, I'll give permeating mass and nebelgast heral a try in the main, cause they can be either super good or good/medium ( a friend of mine, a very skilled player, told me of an episode where the faced Mass in modern and that wasn't that easy to navigate through it. this is all i know about the card), so this makes the cut for a couple flex slots, in my opinion. Not sure about executioner, I'm more into the single kira maindeck.
As for GoST, I think eladamri's doens't solve the problem of the turn it starts attacking. you either hierarch into gost turn 2, or it's a slow walker. I see it as a side option at most against non creature decks, once again.
Let me share my first idea, it's just something written on paper and side is just brainstorming, so take it for what it is:
As a note, I never play givers, thalias nor mariners because of a personal choice. I normally go full fliers and flash, so you might want to jam in the current stuff that puts up results.
Collector Ouphe could also be reasonable, but it shuts off vials which are really synergic with calls.
Regarding curious obsessions, the more i play spirits (UW, esper, jeskai) the more I understand this tribe can only work under three lines of play: 1) card advantage and selection and tempo advantage ( 4 mana to put in play more than 4 mana total costs) through collected company 2) Tempoing out the opponent via thalias and deputies 3) having sources of card draw that provide extra resources at least once in two turns ( teferi time raveler draws once into 4 and it's absolutely insufficient as a draw engine). So whenever you are not going for the first two, you have to find an alternative way to draw repeatedly and Obsession has been the best for me so far ( better than any tool like visions, opt, light up the stage, teferi time raveler, hyerogliphic illumination)
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