I too run a Trevenant deck in Expanded format games. Following are a few thoughts for your consideration:
What is the point of having a 1-1 Mismagius line which is somewhat inconsistent in setting up, does not seem to compliment the deck strategy and is an easy 1HKO prize?
Why no Wobbuffet (Bide Barricade) and Necrozma GX?
I run 3 Wally which I've found to provide a very highly consistent T1 Item-lock (along with 2 Tapu Lele GX, 1 Jirachi EX, 3 Trainer's Mail and 4 Ultra Ball).
Your card-draw support (2 N and 3 Professor Sycamore) seems woefully insufficient, IMO. Why no Colress??
No ACE Spec?? I'd suggest Computer Search.
IMO, 1 Head Ringer is really insignificant, especially with all the GX Pokémon now.
Bursting Balloon don't really matter, IMO. The attached Balloon is only temporary and opponents may still incur the damage for better game and/or board position. Also, opponents can easily Lysandre and/or Guzma around your active Pokémon with the attached Balloon.
1 Xerosic seems excessive given the 2 TF Grunt and the 1 Field Blower.
1 Lysandre is insufficient, IMO. What if you your lone 1 is prized?
Why no Brigette? This would be more helpful than the lone Level Ball, IMO.
Silen Lab is really not needed, IMO; Dimension Valley is the preferred Stadium for this deck. By the way, Parallel City could be a consideration should you want/need to remove your EX/GX(s) from your bench.
IMO, 7 energy are also insufficient.
What is the strategy of your deck?
Dark decks are very tough to beat and will have, IMO, an auto-loss. Any thoughts on how to deal with those decks?
So, here are my thoughts on your suggestions...
1. I play a 1-1 Mismagius line due to Zoroark GX being a big contender in most of the decks in Expanded nowadays, so since Trevenant is weak to darkness, Mismagius is my counter. Something I've noticed about Zoroark GX decks in general is that they only play Special Energy, which means that if I can get Mismagius out before they attach any energy (which, even if they do, I have Enhanced Hammer, Xerosic, etc.), I win. Wally can search for the evolution if I need it to.
2. Wobbuffet is interesting, maybe I can find room for one (Item lock with Trevenant is more powerful, but a one of would be nice). I would only play Necrozma if I had DCE in my deck, so I'd have to find room for DCE first.
3. Three Wally seems (at least to me) one to many, but I may be able to throw in a Jirachi EX. I'll try it out anyways though so I get an idea of what your talking about.
4. I play a low draw support since I play VS Seeker, but I could try throwing a Colress since I can take advantage of (opponent's) Zoroark GX banking off benched Pokémon.
5. I don't play an Ace Spec because... the only one I have is Life Dew. I would love to play Computer Search, but until I get one, I won't play an Ace Spec at all (though Life Dew isn't that bad).
6. I agree, I will remove it for something for sure. It hasn't helped at all, so I might as well replace it with something else.
7. If you don't think I should play Bursting Balloon, what do you think I would play? Rescue Scarf, Choice Band, maybe something else?
8. I like the Xerosic in here since it discard Special Energy; I don't have to use immediately, I just like having the option to VS Seeker for it. Tools aren't really a target with Xerosic since I have Item Lock, so between discarding Stadiums (Field Blower) and Special Energy (Xerosic), I'd prefer the Special Energy.
9. One Lysandre is sufficient in my opinion since this deck focuses on spread damage. If I do prize Lysandre, it isn't really a big deal, but I can understand your point. It hasn't really been a problem for me.
10. The one Level Ball has been helpful, but Brigette does grab a lot more and since Trevenant does slow the game down, I may have time to use a Supporter like Brigette. I'll try it out.
11. I'll try playing the fourth Dimension Valley, I have been having trouble with energy, so another Dimension Valley would help with consistency. Silent Lab is like Wobbuffet that I can have along with Item Lock (Trevenant in the active), so another level of disruption is nice.
12. As I mentioned above, I've had trouble with energy so I wouldn't mind another Psychic or Mystery Energy. I do think another Super Rod would be nice so I can recycle more Trevenants.
My strategy mainly consistent of the old way of playing Trevenant with new cards thrown in. It is a little inconsistent, so I would like to remove as much of that as possible. The obvious turn one Trevenant is mandatory, trying to bench another Phantump that turn would be nice as well. Since the game at that point has become a lot slower due to Item Lock, I can just slowly build up an army of Trevenants while attaching energy. So, changes I'm going to make would be...
-1 Level Ball: This is foe Brigette, as mentioned above.
-1 Head Ringer Team Flare Hyper Gear: As I mentioned above, it isn't helpful at all so taking it out would be preferred.
-1 Field Blower: Mentioned under Xerosic.
-1 Silent Lab: Mentioned under Dimension Valley.
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+1 Brigette
+1 Psychic Energy
+1 Super Rod
+1 Dimension Valley
Let me know what you think, I appreciate the advice
@TuxedoBlack, thanks!