if you can make custom portals, make one thats activated by a creeper head being struck by lightning. if youre gonna make the portal frmae like a nether portal, make the portal blocks themselves be blue and move around, like a charged creeper. also make a few small custom mobs in some dungeons or towers that do nothing, like a version of a taller zombie with new textures, a less anoying silverfish, and just random useless things like that. just fun to have them

Make a biome that is basically just a better birch forest. Also one for oak. Like mojang teased about but never implemented.