For my SubTerra like game, I will do detail wise:
Add dirt, rounded walls and expanding pipes from Subterra 1 for terrain objects.
Add rocks, diamonds, bombs, emeralds, and doors, keys that are used up at doors with the matching color, water, and a floaty life saver for non terrain objects.
Add spiders, drakes, knights and the Mountain King for monsters.
For my Mario Bros game:
Do it the way the original Mario did it, but use a different behavior for Bowser than just shooting fireballs and jumping, to make him more challenging
For my Mario Kart game:
Do it the way Mario Kart 64 did it, same power ups and hazards as that game
For my Asteroids game:
Do the same things as the original Asteroids, but also add a new final boss enemy that is even harder to defeat and attacks even more aggressively than the UFO
For my Zelda game:
Make the final boss more challenging, but otherwise it's the same idea as the original 80s Zelda games for the NES or GameBoy
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King Azb's Arena today!