I don't know about the Windows and macOS ports but the Linux version of Icecat could be derived from the Trisquel abrowser source if they are kind enough to allow and encourage us to do so. My comments here should in no way be considered a judgement of either of these ambitious and much needed projects since I understand and greatly appreciate the time investment it takes to keep such efforts alive and active. That said here are a few ways I can see that Icecat may still differentiate itself from abrowser...
1. Provide Mac, Windows, and Android ports. For what should be obvious reasons will likely never have any interest in building these themselves.
2. Retain current fingerprinting countermeasures (I don't know what these are -- I'm just looking at features listed on the Icecat project page [A]).
3. Switch to uBlock Origin[B] as the frontline method for blacklisting unwanted content. When I install this plugin I generally select a handful of lists in addition to the defaults. It may be easier to just document this well if it's non-trivial to pre-select the defaults on install.
4. Implement several of the about:config changes recommended in Trisquel's documentation[C]. Some of these are anti-malware services but uBlock has several options that can be aggregated to serve the same purpose using crowd sourced data instead of vendor sourced data.
5. Continue to maintain Icecat's list of addons[D] or combine efforts with Trisquel's[E] since it appears to be yet another duplication of efforts.
6. Flesh out Icecat's current documentation[F] which -- despite being somewhat sparse by some standards -- is still considerably more than what Trisquel has listed for abrowser[G].
7. Consider switching to a default privacy focused search engine such as Ixquick[H].
8. Consider if the current list of default plugins[A] is still relevant. The call for bringing back Firefox's cookie management feature, for example, may be mitigated by using a combination of plugins such as uBlock Origin and Privacy Badger[I]. If not, then perhaps there's a suitable free plugin that achieves similar functionality without being an administrative burden on the average user.