Another crazy idea before I dash off to work, this one about as HAYO as can be.
"A high-tier boat capable of cruising in lava (and potentially other hot liquids)."
So Minecraft finally fixed boats around 1.9-1.10, and they're ridiculously amazing, even in SMP - which still blows my mind, frankly. I've been faffing about with various things between a server with friends and an IC2/JEI-only pack for my own testing, and one of the things I looked at was boats. The rubber and carbon fiber boats are super-speedy (like any other boat now) - nearly indestructible, actually; I've been trying to break them without much success. They are not lava-proof, however, and will go up in smoke when you place them in it.
The electric boat, however... floats in it.
On the downside, it's slow. This is ameliorated somewhat by using EU on your person, but even then you're only /barely/ exceeding the speed of a normal boat - and also, if you're in lava like a crazy person, you're constantly on fire. Additionally, when you break it to retrieve it, lava will instantly destroy it, as lava tends to do to things.
If there was a way to 1) prevent heat damage/fire ticks/whatever, 2) actually retrieve it from lava somehow - maybe shift-rightclick it straight back into your inventory? and 3) ideally tweak the speed to go a little faster, I think we'd have something really amazing on our hands. Of course, it'd be expensive to craft I'm sure (iridium makes for indestructible reflectors, why not an indestructible boat?), but cruising the Nether like a boss would be completely worth it.
(presuming you're referring to fire damage from the lava to the player, rather than to the boat here)