Ships remaining that don't have a variant: Adder, Anaconda, Beluga, Dolphin, Federal Corvette, Fer-de-Lance, Hauler, Imperial Clipper, Imperial Courier, Imperial Cutter, Mamba, Orca, Sidewinder & Vulture.
The Hauler is already a nice starter exploration ship, and the Anaconda, well, say no more. The Fer-de-Lance and Mamba are effectively variants of each other. Realistically I can't see the superpower navy affiliations getting a variant given the ARX situation. I'd also say the passenger ships are unlikely to get variants, although that's not to say I wouldn't like to see an Orca with less windows and more exploration utility. That leaves us with: Adder, Sidewinder & Vulture.
The Vulture could make for an interesting exploration variant. It's traditionally a combat ship, most people shun the idea of it doing anything else, so a variant would work in that sense. It's got a decent amount of internal slots that can be re-shuffled to accommodate exploration focused modules. Downsized weapons to upsize the FSD, plus SCO optimisation. The small footprint lends itself to exobiology. It's main rival would be the DBX, which has limited internals to match a fuel scoop to it's FSD. A Vulture with a bit less range, but better module sizing options would be a tempting prospect.