Yes, by all means everyone can have their own personalized copy of the manual with a customized cover. :)
And yes, Opentoonz already is an alternative to Adobe Illustrator for me and I must assume others.
I have Adobe CC and I haven't cracked open Adobe Illustrator hardly at all since I started using Opentoonz three years ago.
(and somewhat further.. I prefer Coreldraw over Illustrator so would likely use that if I needed to use an external vector drawing program).
Obviously, there are things a dedicated vector drawing program can do above and beyond Opentoonz capability but even there we can use workarounds to achieve similar results.
An example of this would be the various shape merging tools; combine, merge, intersect, etc.
If I have to do a lot of this I would do it in Coreldraw.
Similarly, shape organization tools such as Align and distribute cannot currently be found in Opentoonz.
Such tools can be important to -illustrators- moreso than animators and are of course great timesavers over aligning and distributing shapes manually.
Returning to the cover for a moment...
Those that have Adobe CC can easily add a front and back cover using Adobe Acrobat as that is the original PDF program.
It's been awhile since I've looked at alternatives but I'm sure there are programs out there that can do the job of adding covers to the currently generated PDF.
We'll have to talk to Gab3d and others that are currently maintaining the documentation and see if they might add a cover page via Read the Docs.