I guess here is where I would say I would draw the line on things, is on what a person while trying to survive could reasonably work out on their own and produce.
Things like nukes would be essentially impossible for a person to work out as they not only have to (re)discover the mathematics required, but also would have to rebuild all the supporting physics theories on atoms and sub-atomic particles plus all the practical knowledge of engineering, explosives, and material refinement/enrichment. Microscope and Test tubes are not going to cut it.
The character will already be hard press to be able to make an internal combustion, electric, and steam engines, and are at the very far end of the realm of reasonable. A gun is a lot simpler prospect by comparison in design and know how. I would see a rifle as much further in the realm of possibility, of a single person's capability.
On to much more interesting prospects, I would love to see a full fluid/gas management system in the game with pumps, pipes and reservoirs. I think given how blocks/objects and the terrain already work, being able to bury pipes and wires should not be an issue. Open flowing water and such will be a lot more complicated. I would love to see infinite water sources be limited and unmovable, so that the water has to be transported by pump, pipe, and/or aqueduct to where you want/need it. That lakes and ponds have a finite amount water, that if not feed by an source(Source block, rain, snow melt, etc) would eventually run dry. I know that there are some major technical/programming challenges to implementing such a system.
I like the idea of simple mechanics that allow the player to create complex systems, so I am hoping for more than just muscle power and electric powers. Mechanical power would be really cool with ropes, pulleys, and gears hooked up to windmills and waterwheels, that would eventually connect to more modern motors. I could see survival being broken into two trains, progressive where you are progressing through the technology in a tree format to learn and expand your capabilities. And the second survival mode references as abandon/lost, where you have modern know how, you just have to work out in a more web fashion the nuances of producing specific technologies in a much less structured progression.
I do like the ability to scare off predator animals, but some of the monsters that are a few notches higher on the intelligence scale, maybe just as likely to be attracted to fire. I can see the PvE threats running the whole gamut, with players needing to research/learn how to deter/combat a variety of threats and behaviours of the different elements.