Why to include? Alva is an OpenSource Gui / Website designer based on Electron with a modern minimalistic GUI. It could replace Adobe XD for those being new to Solus. The Repository doesn't include many options for graphic / Gui design on Solus. The project gets updated often and the developers respect the community and fix bugs reported by the users.
Open Source? Yes it is.
Source (.tar.gz): https://github.com/meetalva/alva/archive/v0.8.0.tar.gz
Currently considered beta (on their home page). Waiting for a stable non-beta release. But this seems rather handy and something I'd end up using instead of Inkscape (for mockups), so consider me the maintainer when the time comes.
But it's based on Electron, sadly, and I think it'd be better to include an even more comprehensive tool where development has progressed a lot more
I don't agree and unlike Pencil, this is actively developed.