Nothing, just an adblocker. No bookmarks, history, group tabs. Literally, an address bar that also searches with one default engine.

Must have:

* cross-platform: Windows, Linux, Apple, Android.

* handle great majority of HTML/CSS "features" handled by the mainstream browsers.

* easy to use bookmarks, allowing sub-folders within folders (see below).

* allow position within each bookmark folder to be remembered on next view.

* ability to seamlessy synchronize bookmarks between versions of the brower on different machines, preferably without having to create some sort of account to do so.

* an effective ad-blocker.

Should not:

* include calendars, email clients, etc.

* have hard-coded bookmark folders like "mobile". Populate the bookmarks with top-level folders like that if you must, but allow the user to delete them. I want to organize my bookmarks my way.

All of the above is just off-the-cuff points I remember. Given time I could probably dredge up a lot more suggestions. Like treating bookmark views almost as a web-page, so you could open a bookmark with either a click (replacing the current view) or as "view in new tab". Opening a bookmark with a click would show the new page and clicking on "back" goes back to the original bookmark view.

Open source.

Ublock Origin & NoScript equivalence built in.

"Reader mode" like Firefox reader view but traversing links doesn't reset you back to non-reader view.

An extensibility framework of some kind.

- Not Chromium based, there are too many of those already.

- Effective ad-blocker, cookie notification auto-decline, and general annoyance blocker.

- The ability to set my own search engine via a simple query string. No need to build anything fancy or pick for the user.

- I’d have a hard time using a browser without my password manager being able to integrate, which would mean 3rd party extensions, as much as I hate to say it.

- Cross platform (including mobile with sync), if it’s going to be a daily driver. Mobile really complicates things these days when talking fast, simple, and private.

- But most of all, don’t listen to me or all the comments here. Keep your vision pure. Most browsers start off simple and fast, which is why people start using them. Then they carelessly add feature after feature in response to user demands, the vision gets compromised, and they end up another bloated mess. Be bold enough to accept that your browser isn’t for everyone. It will have its true fans (if it’s good), and for people who want all the features, other browsers exist for them.