Here are some sites to help with inspiration for your spectrum Sequel:

The top 50 underappreciated ZX Spectrum games
http://www.denofgeek.com/games/24911/the-top-50-underappreciated-zx-spectrum-games

30 best ZX Spectrum games
http://www.techradar.com/news/gaming/30-best-zx-spectrum-games-594151

The World of Spectrum Archive
http://www.worldofspectrum.org/archive.html