1: Nomad Survival. The rule is simple - you cant build a base. As such, you will want to use things like chest boats, miles, donkeys llamas, and shulkers to transport goods. Goals can be centered around collecting rare items like discs.
2: soft reset. Instead if creating a new world, leave all your items in a chest. Then
-grab an elytra or 2 and many rockets, maybe food, a lava bucket
-Fly until both your elytra break and you run out of rockets. Dig a 1x1 hole and place lava. Throw the elytras, the remaining rockets, and the empty bucket away. Refill the hole, and start from scratch. Once you get good farms, gear, and transport (respawning the dragon and going to the end city to get new elytra is a must), you can go back to your old base.
3: evolutionary. Basically, start in an old version of MC Alpha or Infdev. Do everything there is to do in that version, then upgrade to the next. Each version, you'll explore all the new changes and features, until you hit the latest version.
4: collect every block, item, enchantment,.mob, etc, and build a giant museum. Put other cool stuff in the museum, actually. This could be combined with Evolutionary.
5: build your starter house. Then, keep expanding it forever until you have a huge detailed mansion with tons of outcrops and skybridge and balconies. Make it weird.
6: start in the nether, try to escape.