What are you doing as Panth? I'm guessing lane Pantheon.
Assassin or Bodyguard?
You can still rush brutalizer for that powerspike if you're going Assassin. Ghostblade isn't bad on him, especially early on.
Tiamat can be a replacement powerspike if you're not planning on going Ghostblade. Hexdrinker if going against magic damage. Basically I just stack mid tier items on the way to LW. So for this new BC, if I wanted to be kinda well rounded, I would go.
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and this is the build I use for the most part (BC change = Brutalizer-->Phage)
an assassin build would go more like
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I don't really play Assassin Panth as it relies so heavily on snowballing, but it should work fairly well. IE works with Panth's passive and his Q. Yomuu's will help you stick to targets and boost your AS. I would also recommend picking up an Elixir or two on your way as it works with all physical damage (abilities included). If you don't like the IE on Panth, you can still grab the new BC on an Assassin Pantheon build as it gives good AD and a lot of CDR which allows you to use your abilities more often. I used to run a full pen build starting with GB and a second Brutalizer, finishing up the BC late in the game after LW. With the new BC, you can do pretty much the same thing, except you'll be sitting on a Phage instead. The Rage passive is also really good for sticking to targets, and the added HP allows you to survive a little longer in a scrap.
Jungle Pantheon will look like this at full build:
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and I usually call this one the Bodyguard as you will be primarily peeling for your carries in teamfights, so you want a lot of health and resistances to keep you healthy while taking on the Bruisers and Tanks trying to engage on your ADC. You can go for an Assassin build in the Jungle as well, but you are more effective for your team if you build tankier.
Pantheon works with a lot of different items if you feel comfortable with how you are using them to compliment his playstyle. Armor pen is a great stat on Panth, but Last Whisper will always be a better item to rush than the new BC. BC is and is going to be something that will be more effective later in the game when you can reduce armor of enemies for yourself and your allies, which is why traditionally, no one finished their Brutalizer until their 4th-5th item. Phage will act similarly as an item someone sits on for it's sticking potential, and will later finish into BC after they get more core items, and enemies armor values get higher. The early powerspike in Brutalizer will be missed, but if you run Ar/pen reds and go for an early Tiamat, you reach a similar powerspike and your burst damage will actually be higher. The downside is that you will need an extra 500-600 gold to get Tiamat. An extra benefit of going this route is the added waveclear that it provides and you can save your mana and CD's to harass the enemy.
Anyway. Hope this helped. Try out some different builds and see what you like. Good luck out there. Peace.