This might be a dumb question, but is it a viable solution to the Israel-Palestine conflict for Jordan and Egypt to respectively annex the West Bank and Gaza Strip as they did in '48?
No, it would not be viable. However, it isn't a stupid question. Some older plans, like the Allon Plan proposed shortly after the Six Day War, involved Jordan and Egypt annexing parts of the West Bank and Gaza, and this wikipedia article indicates that there are some supporters for a "Three-State Solution". However, such a solution is not feasible. Every party involved would prefer a two-state solution.
In 1979, Egypt renounced its claim to the Gaza Strip as part of a peace agreement with Israel, in which Israel returned the Sinai to Egypt in exchange for peace and Egypt recognizing Israel. In 1988, Jordan also renounced its claim to the West Bank, saying that it should be controlled by the Palestinian Liberation Organization (PLO). Israeli Prime Minister Benjamin Netanyahu also, endorsed a Palestinian State, and the Oslo Acoords signed between Israel and the PLO called for final status negotiations to create a Palestinian state.
Jordan and Egypt do not want administration over the Palestinians. During Operation Cast Lead, an Israeli ground oepration against Hamas in the Gaza Strip in 2009, the New York Times reported that Jordan and Egypt were fearful about being pressed to deal with the Palestinians and with territory in the West Bank and Gaza:
Egypt and Jordan fear that they will be pressed to absorb the Palestinian populations now living beyond their borders.
“Gaza is no longer Egypt’s responsibility, and Egypt is determined not to take it back,” said Abdel Raoud el-Reedy, a former ambassador to the United States who is the chairman of the Egyptian Council for Foreign Affairs.
While the prospect of having to absorb the West Bank may be remote, Jordan does not want to have to do so, fearing it would destroy the fabric of society in the country, where about half the population is of Palestinian origin.
Furthermore, the Palestinians themselves are strongly in favor of their own state, either over the West Bank and Gaza, or over all land west of the Jordan River (including Israel). When polled, Palestinians in the West Bank and Gaza expressed support for either a two-state or one-state solution:
Respondents were asked about US President Barack Obama’s statement that “there should be two states: Palestine as the homeland for the Palestinian people and Israel as the homeland for the Jewish people.”
Just 34% said they accepted that concept, while 61% rejected it.
Sixty-six percent said the Palestinians’ real goal should be to start with a two-state solution but then move to it all being one Palestinian state.
Given that Egypt doesn't want to annex Gaza, Jordan doesn't want to annex the West Bank, Israel has agreed to a two-state solution, and the Palestinians want independence, a Three-State solution is not viable.