idk if you have it but being able to craft reinforced deep slate would be cool cause you can break it but not obtain it. I would probably craft it out of deep slate... maybe the dragon egg? I would do something like 4 obsidian and 4 bottles of dragons breath around the middle and put a chicken egg in the middle. it would be pretty cool if you could get multiple dragon eggs! (I also wouldn't have to go and kill my annoying friends just to find out its in their ender chest) crafting blaze rods would also be cool. idk if you have it or not but I would just put a stick in the middle and surrounded it with gold nuggets and maybe a lava bucket at the bottom. blaze rods kinda look like gold sticks... and for the heavy core.... it looks like a mob head so... any mob head (excluding the dragon head cause its so chunky) in the middle and surround it with 4 iron blocks and 4 obsidian. that would still make it easier to get lol. maybe spawn eggs crafted from a mob head in the middle and then surround their heads with their drops. for stuff like cows do raw beef in the middle and surround it with leather. if you don't have the elytra I'd just do a phantom membrane surrounded by a bunch of feathers... or the opposite.