Interesting! I have been thinking about a similar "average Germanic" language, but I don't really have enough linguistics background to pull that off yet. Also, I have very roughly compared the Slavic languages to see which one I could learn to be able to communicate with people of most Slavic languages[0] and decided that Slovak was the most "average" language so I am planning to learn that. Too bad there's no Slova…
What would that Germanic language cover? German, Dutch, Flemish? That's it, right? English is out, because that's half French, and the Scandinavian languages have drifted too far from German to be even half-assed mutually intelligible, no?
About mutual intelligibility with Scandinavian: quite a few basic words are identical in pronunciation between e.g. Swedish and Dutch, and many are similar enough that when speaking slowly you can get reasonably far, I would think, although I haven't tried that much since in practice you fall back on English all the time. When you look into the dialects there's even more you can find that's very similar.