My idea is that the player could build a portal, not sure which blocks would fit best. That portal would spawn him in front of a huge mountain, which he would then have to climb, fighting new mobs on the way to the top. Once he gets close to the flat top, the big yeti, the final boss in this dimension, would roll big snow balls down the mountain, kinda like the original donkey kong. When the player reaches the top, he has to fight the yeti who tries pushing him off the top, in which case the player would have to climb it again, depending on how far back down he fell. Once the yeti is defeated, he drops a lot of xp and one or more rare, uniqe items before a portal spawns in the centre of the flat mountain top, giving the player the oppertunity to travel back.
This is a good idea, but it would be nice to expand the dimension with either a slope off to the void, terrain with similar mountains elsewhere, or both.
In any case (i.e. inside the mountain(s) if void), some unique ores and/or select vanilla ores would be nice if they were to generate (as an incentive to go there and explore) and give loot if created, as well as some dimension-specific structures.