So what genre of video game would capture the look and feel of Nausicaä best? I'm pretty sure more than a few people would choose a RPG format. Not a bad choice. Nausicaä does have a lot in common with certain JRPGs like Skies of Arcadia or Final Fantasy. However, there are a couple of major problems. For one RPG gameplay depends heavily on leveling up by killing foes, particularly monsters. This necessity of the genre doesn't mesh well with the setting since, in Nausicaä, slaughtering lots of people would be antithetical to the message of the source material. Plus, it's made abundantly clear that killing the giant insects of the forest is at best extremely risky and at worst tantamount to suicide.
The other major problem is the dependency of RPGs on magic to keep things interesting. For better or worse though there is no magic in the world of Nausicaä, only relatively rare psionic powers such as telepathic communication (used more for narrative expedience rather than visual flare). Even the younger of the two Dorok emperors, who is said to be a powerful psionicist, could only perform limited forms of astral projection and telekinesis. An alternative that some RPGs embrace is to make sci-fi technology so far in advance of what exists in reality that it is functionally the same as magic. Again though this wouldn't jive with the setting since the advanced technology of the old world is no substitute for natural human ability. The elder Dorok emperor wears a special helmet that allows him 360 degrees of vision, but he still fails to spot Kushana's coup d'état until it's more or less over. The God-Warrior can spew forth fiery death, but is difficult to control even by the one it thinks of as its master. Even the prized secrets of the Crypt of Shuwa are the kind of technology that only extends life or creates "Heedra" (basically a kind of troll), often with body horror inducing side effects. Any attempt to directly harness the power of the Sea of Corruption results in the "Daikaisho," a rapid expansion of growth brought on by swarms of irate insects carrying Toxic Jungle spores. Even the Master of the Garden (arguably the single most powerful character in the manga) is only able to restore Nausicaä to health by having her rest in a special pool for an undisclosed amount of time.
For these reasons I don't think a RPG is feasible, but obviously other options exist. How about a flight-sim or point-and-click adventure game? Well...both capture certain aspects of Nausicaä but completely leave others out. Side scrolling platformer or RTS are ideas that come to mind as well, but again, any such game would be so far removed from the source material it might as well be its own IP.
I want to stress that this need not boil down to a bunch of boring fetch quests and tedious escort missions. For example, how about this? The player pilots (or guides) a bumblecrow past clouds of giant insects and poisonous miasma to a crash site in the Toxic Jungle. There the player must find survivors, salvage engines and negotiate with wormhandlers who have also staked a claim on the wreckage. Rather than a simple pass/fail state, greater degrees of success nets more resources for the player to draw on in the form of better aircraft, new equipment, safe havens, recruitable followers, and fresh supplies, not to mention renown.
There are a huge number of interesting places to visit both old and new; the Tolmekian Capital of Talos, the Colony City of Nosimo, the Westernmost City of Epo, the necropolis beneath Pajite, the "mining" town of Semo (actually cannibalizing the hulk of a derelict starship), or the River Taria which flows deep within the Sea of Corruption. Then there are the various locals in the Periphery too like the Valley of Sand and the Sea of Salt. The latter of which provides fresh winds that keeps the Toxic Jungle spores at bay.
original Foundation trilogy by Issac Asimov, Ursula K. LeGuin's first trio of Earthsea novels, and Dune by Frank Herbert. The influences of Dune are especially apparent with regards to sandworms and Ohmu, as well as Fremen and The People of the Forest. Some of Mr. Miyazaki's own works such as the animated film Castle in the Sky or short manga Journey of Shuna are rich with concepts that could be re-purposed for a Nausicaä video game. Of course really history works as well. It's easy to see Nausicaa as a Joan of Arc, or Kushana as Elizabeth the I of England. Perhaps Chikuku is a Gandhi figure, encouraging tolerance and nonviolent resistance as a tool for change.
Ultimately the best part of making Nausicaä into a game is the chance to explore the setting and characters in more detail. Everything from the statesmanship struggles of Kushana and Chikuku as they try to rebuild their respective nations shattered by war, to the ultimate fate of Nausicaä herself. Did she eventually make the journey to the pure land? And what of the proverbial gatekeeper, the Master of the Garden, whose ark-like residence lies hidden in the Goss mountain range? Nausicaä set people on the path of reconstruction, but it is still up to each and every one of them (including the player) to restore the world to its former splendor (hopefully this time they'll be a bit wiser too).