I played GSZ BUG at NYSE and have been messing around with it online as well. Here's the targets I've used/considered vaguely in order of usefulness:
- Leovold - Card is amazing, GSZ for it is amazing. Since most blue decks pack ways to kill this, I always run 2x since getting one to stick is huge. It's also nice that GSZ for Leo can't get Pyroblasted, so you can at least get your card out of it.
- Deathrite Shaman - ramps you into bigger Zeniths, helps ramp/fix for Leovold. I play 4x basically for that reason alone, and sometimes you do want to Zenith for them.
- Tarmogoyf - having ready access to a dumb beater is great, but this card is not always good (since you only want it in situations where you need to close out a game or where creature combat is relevant). Being able to have an effective 3 with Zenith when it's good, while only needing to run 1x puts it over the edge for me. Grim Flayer is an alternative that feeds itself, but doesn't hold off Eldrazi as well.
- Scavenging Ooze - surprisingly good as a 1x of for hating + beating, since most decks either have creatures or recursion. This is better with Zenith for similar reasons to Goyf.
- Dryad Arbor - I haven't ever super wanted to have Arbor except against Shops, so I've been sticking to 1x in the board to get a leg up against mana denial decks.
- Manglehorn - I was initially really excited for this, but found it pretty disappointing. Destroying an artifact is not insane against PO, and this dies to most things that kill Leo without quashing Outcome itself. I'd just play Reclamation Sage over this if the ETB was something I thought I needed.
- Trygon Predator - steady flying clock, good against walkers, good to get down against Oath/Standstill/Shops/slower PO decks. I've been very impressed with 1x main.
- Ramunap Excavator - 1x is fine, but I haven't been super impressed. In general I've felt like either Leo or Trygon is a better angle of attack, but I need more testing to have anything conclusive.
- Caustic Caterpillar - okay, yes, I know, but hear me out. Reclamation Sage can be a great card, but is not a great silver bullet against Shops. If you're casting a 4 mana sorcery against Shops, you are probably doing just fine. If you're under pressure but have mana, Caterpillar costs exactly the same to Naturalize something (whether Zenithing or hardcasting), and you're probably chumping with the Sage anyway. This gives you a way to try and crawl out from under lockpieces and stick something more serious. I'm not yet sure it's worth the SB slot, but getting a Shops player to sigh loudly and say “Phyrexian Revoker naming your stupid little caterpillar” is really its own reward.
- Glissa, the Traitor - owns combat completely. If I'm worried about White Eldrazi or big ground threats like Wurmcoil, 1x in the side can do some amount of work (but I will admit it's got a pretty limited scope).
Other GSZ BUG considerations if people are interested:
- 2x GSZ seems correct so far if you're trying to build around the card. it's expensive to cast if you want to have a big impact, and you want to make sure you have a big enough density of actual threats and answers in your deck. On the other hand, you do want to consistently see the card in longer games, as it really is a powerhouse once you can tutor any creature in your deck into play.
- Blue spell count for Force can start to be a problem. Part of the reason I like Trygon over Rec Sage or similar is that prioritizing GSZ and targets really chips away at your blue count, and I found myself choosing cards just to get more blue spells. I really would like to find some good 2 cost U/G targets to help address this.