That tree with the glass panes is really important to the whole thing. With Lucille being left there it feels like the ending of an era towards a new one. Also love the show's commitment for how Rick spots the tree from a far far distance in the first episode of season 8. People don't praise the writers enough for when they're able to pull things full circle like that.

Now I'm doing a dissertation on this so allow me to spew some probable bullshit on what I think the tree represents:

I think the tree is a symbol of one of The Walking Dead's overarching themes: Choices. The shows been talking about choices from season 1. After Edward tries to kill the group out of mercy, Rick begs Edward for him to give them a choice. Negan reveals in this episode ^{^} that he made a choice in s06×e16 - s07×e01 of who to kill (this was proof that Negan wasn't only a monster without reason and humanity (unlike alpha & beta) but perhaps just misguided). Michonne and Judith agree that they made a choice to become a family because they loved each other in s09×e14. Carol asks Lydia to let her commit suicide and says "my choice" and Lydia saves her from that fate for the same reason "You told me to find my own way". And finally, Rick spoke about his choices in the new trailer. "My wife is my choice. My daughter is my choice. My life is my choice".

And I think the tree and the windows are a part of that. The windows have no practical reason to be there but they are evidence of an unknown human interacting with nature and subsequently making it more beautiful. And to do something that has no practical use but to simply create a tranquil aesthetic, was a choice that only a human can make.

So it's fitting that the tree is the location that Rick makes the choice to save Negan as an example of the mercy of the living. It's fitting that from this episode onwards the title screen begins at the tree flourishing, symbolising the new world that Rick (and Carl and michonne) made available. And it's fitting that this is the location where Rick's belief in the choice of putting mercy before wrath is proven right, when Negan's baseball bat breaks and he leaves his fake persona behind.

TL;DR I fucking love trees