Ports:
Sonic Advance Trilogy (Is not on modern systems, could use a widescreen remaster)
Sonic Battle (Is not on modern systems, could use a widescreen remaster)
Sonic Heroes (Is not on most modern systems, PC port is severely outdated and largely inaccessible)
Shadow the Hedgehog (Is not on modern systems)
Sonic Riders (Is not on most modern systems, PC port is largely inaccessible)
Sonic the Hedgehog 2006 (P-06 has this covered, but an official endorsement and availability on modern storefronts would be appreciated)
Sonic and the Secret Rings (Is not on modern systems)
Sonic Unleashed (Is not on some modern systems, especially PC)
Sonic and the Black Knight (Is not on modern systems)
Sonic Riders: Zero Gravity (Is not on modern systems)
Sonic Generations (Could use a small remaster, accessibility is dropping and the PC port could use an update and better config tool)
Sonic Lost World (Is not on some modern systems)
Remakes:
Sonic Rush Duology (Could use some reworks and graphical enhancements for today's market, accessibility)
Sonic Colors DS (Colors Ultimate DLC anyone?)
Remasters (Large scale):
Sonic the Hedgehog 4 Duology (Episode I could use some graphical tweaks to appear more consistent with Episode II, Both episodes could use some physics tweaks and level design changes to feel better to play on console/PC, Episode Metal needs an entire overhaul)
As an additional note, Sonic Adventure 2's Steam port could use an update. The port is pretty poorly done, resulting in noteworthy issues such as frequent crashes.