Construct the top tier Beacon pyramid. 164 diamond blocks should keep you busy for a bit
I'm also a proponent of installing as many mods as possible. Really extends the game out
I run on a Mac so mods are a bit troublesome for me, but the 164 diamond blocks is a challenge I'd definitely pursue, no kidding. Having a beacon like that would be good for if they ever add any other boss mobs in the game.
The wither should keep you occupied for a while, considering how long it will take to get the materials to spawn it (roughly 120 Skeleton kills in nether fortresses to get the 3 heads)
I'm almost there, right now I'm just farming mats for potions and enchants to get the best possible gear possible for the fight. Right now I have full Diamond armor w/ enchants, a Bow with Power III and Punch I, a Diamond Sword w/ Sharpness V, strength potions, potions of instant health II... just a matter of making some regeneration/swiftness potions and getting my 3 skulls and getting some better enchants on my armor and I'll be able to take him down.
A lot of people say to kill Wither first and then the Nether Dragon... but I believe the Wither is a much harder boss than the Dragon. If your smart, you can kill the Nether Dragon without taking a single hit from him. Just avoid looking at the Enderman, dodge the Dragons fly-ins and kill his health beacons and your set.
Just an idea for fun, build a beacon on all pyramids you have found so far and by using a map, try to map an X. Before going to the center, have a final beacon and build a giant pyramid below just to make it seem like the aliens decide to come. Well that would increase the diamond grind i guess?