As someone who grew up playing all the old games, I would love to see some cosmetic unlockables that reference previous games. I’m thinking skins for the ship & chickens to make them look like they did in the first game, the original space background, maybe even an option to make the GUI look like the original. You could do the same thing for the other games too, and some obscure ones like the police ship from The Next Wave would be awesome, too. Depending on what the current plans for the music are, how about unlocking some of the old music tracks? The main theme from TNW is my jam, I’d love to complete a difficult mission to unlock that track.
Some other cool references could be a Planet/Star System/Mission with the wave pattern from the original game, and maybe one for each of the other games that has lots of its iconic waves. A Boss Rush mission for each game would be awesome too, or maybe just a single mission with the best fights from all the games. While I’m throwing out ideas, how cool of an easter egg would a faithful recreation of the original game be? Maybe it could be on an arcade machine floating just outside of the Space Burger.
Anyways, these are the ideas I’ve had so far. I’ll post more if I think of any, and if anyone else has some ideas, please post a reply!