Some small browsers exist that use webkit/webengine for the heavy lifting. Two notable examples even have vim-like interfaces: qutebrowser and jumanji.
Extending them _might_ be easy, depending on the quality of the existing code base. We also need to evaluate these projects for maintainability: they're obviously going to have much less development power.
* Kerning/font presentation code? (text in qutebrowser looks bad on Windows, don't know why)
* Cross-platform OS shit
* Firefox sync is neat and would be missed.
* safebrowsing?
* how much security stuff in engine/vs browser?
* webm, webgl and similar? Presumably handled either by the engine or externally, but maybe picking and maintaining link to external thing is expensive.
* flash handling?
* What UI stuff are we not replacing?
* developer tools (neat, but no reason for us to re-implement).
We also lose access to the existing addon/extension repos. Maybe if we implemented webextension support in our own browser we'd get them back? Don't know how difficult that is.