Not necessarily top of my wish-list personally (that'd be either Kalderash or Welsh Romani) but a genuine and very serious suggestion for when the course-creation tools are at the right point: Channel-Island Norman French of some sort -- Jerriais or Guernèsiais.
I never got around to commenting on the "I want to be a Welsh speaker because..." thread, because I felt I had reasons that were too many, and too unclear even to me, and too hard to put into a single sentence: but in the end, one big reason is that, given that I get compliments on my Catalan in Catalonia, and could speak Occitan to the last few speakers before it died out in the village where my parents live, I felt a certain shame in not ever having managed to learn any of the indigenous non-English languages of the British Isles.
Looking at what I've seen online about, say, Guernèsiais, I would think that there are many ways in which it would be quite a good 'fit' for SSi. It's another endangered non-English language of our archipelago: as for being indigenous, although it can't claim the 3000+ years of Celtic, it's probably been in place in some form for at least as long as English (assuming the Channel Isles were inhabited by colloquial Gallo-Latin speakers around the time the Angles arrived). And, unlike Auregnais, it aten't dead yet: it still has a few, increasingly elderly, native speakers, and an active community of younger learners and would-be speakers, although the materials I've seen online to help them are clearly not a patch on SSi methodology. Oh, and anyone involved in the Guernsey Language Commission has got to be utterly bilingual in English, at least, so getting them on board with the methodology and the course creation tools ought to be relatively uncomplicated, too.
But I have to admit, part of its attraction for me is simply the way it sounds: if you watch this lovely video of a couple of old boys discussing how one of them got delayed by a fallen tree on his way to a Pure Patois evening you might see what I mean. It basically sounds exactly like how I'd imagine people would do talking mediaeval French with accents that keep veering between the West Country and the Black Country. It's mad, and utterly charming.