I started to work on the Doppelcaster but my brain is still busy with the Melobaritone idea... I did put some more thinking on it, and updated the project (I want a new instrument, not just a Melobar copy).
First I would do it with an aluminium neck - there's no need to build something as complicated as a regular wooden neck with a truss rod, and I've always been interested in alu neck, so building an alternative guitar is a good opportunity to try it. Also I would put a Floyd Rose trem (with an headless system) - I used one for the first time last month and really enjoyed it.
One would object that a trem on a slide guitar is superfluous, this is true if you play old school blues, but for me it's more like a two-in-one instrument - for a noise maker such as I am, the sound of extremely distended strings is very interesting, and since I wouldn't purchase a guitar just for its Floyd Rose, I'll take this occasion.
I could easily put in this project all the gear I bought for the neckless guitar - it's been on stand-by for months now - and make it my next project after the doubleneck one...