1: a massive bossfight that you fight from the inside out, or a boss that's similarly fought like starbound's final boss.
2: dungeon system with scaling enemies
3: bosses actually scaling with difficulty.
4: extra boss phases on certain difficulties (for some "But its not over yet!" vibes)
5: entire secret boss when reaching end of game when starting on hardest difficulty and never staying off.
6: endings depending on which bosses/optional bosses that you defeated :
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6.1: bad ending for switching to peaceful from other starting difficulty (overridden by good ending and above
6.2: neutral ending for killing only required bosses
6.3: secret ending for insta-killing final boss (if there is a final boss)
6.4: creative ending
6.5: good ending for killing at least 2 optional bosses
6.6: best(?) ending for killing all optional bosses
6.7: TRUE ending for killing all optional bosses on hardest difficulty
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7: d o o r s
8: funny easter eggs