We've come a long way in terms of more sophisticated variant concepts from Orion ... mini-mag Orion ... anti-matter catalyzed micro fission ... all sorts of ideas. Heck, there are even non-nuclear variants using kinetic impact powered explosions rather than nuclear powered.
Ironically a lot of these advances in ideas come from fusion research, rather than fission research. Fusion research gives us experimental data on magnetic plasma containment, plasma compression etc...