Flathub offers a beta channel for Inkscape. The version that is available is not fixed to 1.0beta2, but will usually be updated to the latest beta release. You can learn which version has been used to create the latest flatpak from the build list at https://flathub.org/builds/#/apps/org.inkscape.Inkscape~2Fbeta .

To add the standard and the beta channel to your user flatpak sources, enter the following in the command line:

flatpak remote-add --user --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak remote-add --user --if-not-exists flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo

To then install the Inkscape preview version, enter:

flatpak install --user flathub-beta org.inkscape.Inkscape

To run it, enter:

flatpak run --branch=beta org.inkscape.Inkscape

More info about parallel installation of both beta and stable, and about defining the version that is supposed to be included in menus is available at https://blogs.gnome.org/alexl/2019/02/19/changes-in-flathub-land/.