There were several proposed 747 updates in the 1980s and 90s, including a handful confusingly under similar names:
1986 - 747-500: Stretched and powered by unducted turbofans / propfans.
1986 - 747ASP: Pretty much a 747-400 with the shorter 747SP fuselage.
1996 - 747-500X: Stretched to 76,2 meters and a new 777-derived wing with a 77 meter span, new bigger engines, 4 nose wheels and 20 mains.
1996 - 747-600X: Same as the above, but stretched to 85 meters.
1996 - 747-700X: Same as the -600X, but with a wider fuselage.
2000 - 747X and 747X Stretch: Based on the 747-400 fuselage and a stretch at 80,2 meters. Wing was based on the -400, but with the span increased to 70 meters with an insert at the root, ala the A340-500/600. Powered by the A380s EA GP7000 or the unbuilt RR Trent 600. The 747X eventually morphed into the 747-400ER, which Qantas ordered.
2002 - 747-400XQLR: Much more subtle upgrade. Essentially a 747-400 with raked wingtips, updated CF6 engines with sawtooth nozzles, higher MTOM, minor aerodynamic updates to the wings and fairings, 2% lower fuel burn and 300 nm longer range. The skyloft was first brought up here.
There was also the initial 747-8 proposal, which was 2 meters shorter than the final product.
You can find more on the secret projects forum:https://www.secretprojects.co.uk/thread ... 523/page-2