Heist.

Heist as a mechanic is thematically appropriate for Outlaws of Thunder Junction, but as a function it is unfair and unfun.

Here are my reasons for this, and a full argument that you MUST separate Alchemy cards from Non-Alchemy formats.

1. Heist as a costed ability : Heist is a fun take on a theft mechanic, and if it were just that I'd like it a lot more. As it has been made, it is way to cheap. Only six cards have this mechanic, but as you are aware Companion was only on 10 cards and were such a problem that you had to at first ban a card in Vintage only to then go back and completely errata the mechanic as a whole. Heist isn’t THAT broken, but each of the 6 cards it’s present on are either costed aggressively or feature a significant upside alongside the mechanic. Grave Expectations and Grenzo, Crooked Jailer are particularly guilty of this; One being a single B instant to either Heist or exile up to three cards from your opponent’s graveyard AND you gain 3 life, the other getting to be A) your commander in Brawl, B) Gets to Heist on entering and each of your upkeeps, and C) Once each turn you get to pay 0 to cast a spell you don’t own. Combined with the other upsides of gaining life and burning your opponent for casting their spells, taking them each turn, and even getting a counterspell make Heist far too much to have to deal with as a player. There’s little to no downside to run any of these in the Historic, Timeless, and Brawl formats which further dilutes the experience of said formats.
2. With Heist being entirely an Alchemy format mechanic, it is now prevalent across all formats able to run them, which by your own design and choice is all but Standard and Explorer. I will freely admit that Alchemy cards bother me, not because of what they do, but their implementation – you have turned it into a parasitic design for all Arena formats and make them aggressively costed, aggressively powerful, or both in order to pump up wildcard purchases and ultimately make these formats not equivalent to what Paper would have access to. I do not doubt what Wizards of the Coast is : you are a business and money must be made. That’s fine. There are ways to enable Alchemy to be its own thing and allow those who enjoy it to play in their chosen format without forcing other players to have to play around them or play with them to compete. I would play Alchemy if that was where I could find these fun designs, but breaking the rules of a format (Brawl and Color Identity for example, Key to the Archive) or making Timeless less Modern+ and more Alchemy+ hurts the community and their ability to transition/continue to play in Paper with decks and archetypes they love. I am not the first to ask for Alchemy to be separated from Timeless or Brawl, but I doubt I will be the last, and honestly I know you won’t. I have to share my thoughts because I know you can do better than what you have done.

What this all boils down to is – Heist is a fun idea but not fun as costed with the cards made for it. Nerfing Grenzo and Grave Expectations to maybe only on entering and becoming a (1)(B) Sorcery could go to remedying the concern the community has about the mechanic. You can also make Alchemy Brawl and allow Brawl to be Commander for Timeless instead.

You will do what you will, but I hope for the best.