Not trying to copy Arc per se; Mozilla did all the work for Nova - I actually prefer it, it feels more grounded into how everyone is used to using a browser. I feel like there's little UX innovation left to be had with the interface of this bit of software.
Saying that, I always like adding extras on top - and I've spent a lot of time making sure users can tweak how they like. I didn't manage to squeeze everything into this final release, but the .1 release will re-expose a lot of customisations that are buried in `about:config` preferences.
The tab sidebar and borders are Arc signature; and Arc has a New Yorkish style blend into it that these browsers don't understand fully. I'm not sure how I can explain it good enough; but it's just that. I don't get the island designs and stuff.