I posted all of this on Minecraft Forums but it didn't get much attention and I thought I remembered something about this being the place to post suggestions so here I am! Note: reading below this line is not recommended unless you're wanting to be spoiled for parts of Sun and Moon's storyline.

So anyway, Solgaleo and Lunala are the first legends ever to have pre-evolutions. Cosmog evolves into Cosmoem and then into Solgaleo in Sun or Lunala in Moon. One of the side characters has the first of two Cosmog found in the game, and once it evolves into the title legend she allows you to capture it since she is not a trainer and it wants a real adventure. In the postgame you can also get another Cosmog by visiting an alternate world that is accessed through the title legends' power to open wormholes. Cosmog cannot breed for obvious reasons. Now, for the mod I'd like to see Cosmog as an insanely rare spawn in the outer End islands, seeing as how its whole "thing" is that it came from another world.

Once caught you can do a couple different things with it. Firstly you can train it, which is rather hard to do since it gets no attack moves until it becomes the legendary. (Maybe just make the evo for Cosmoem time-based since there's really no "version difference" in Minecraft.) Once the legendary has evolved however, it will have the ability to travel between worlds using its wormholes, and can take a mounted player along with it to almost any dimension. (Secret bases would be blacklisted, as would any dimension that is specified in the config to be such.)

Pressing a hotkey while riding the legend would bring up an interface, showing what dimensions are available, and selecting one would open a fancy-looking wormhole for a short time, riding into it on the legend would warp you to that dimension.

Also, if you wanted to add more legends into the base mod besides just Solgaleo and Lunala, consider the Ultra Beasts, obtained in a similar fashion to the games where Lusamine builds a machine and powers it using Cosmog's energy to open Ultra Wormholes in a wide area, causing varied amounts of Ultra Beasts to come through. In the case of the mod, the player would build the machine, and using it would require inserting a cube containing a Cosmog into its GUI, and then Cosmog's health would be rapidly depleted until it fainted. While the machine was draining health it would also be spawning ultra beasts in a wide area around it, and it would stop when Cosmog had no more energy left.

Beast Cubes(the Pokecube version of Beast Balls) should be outrageously expensive, like needs a nether star and four diamond blocks and four gold ingots just for one batch of say five. After all the whole "thing" of the Beast Balls is that it costs millions to manufacture one, according to Wicke, who worked for the people who developed the Beast Ball. Normal cubes would be mostly ineffective at catching Ultra Beasts, but Beast Cubes would be able to do it quite easily as long as the beast was weakened.

Finally, even if none of the other stuff makes it in, I would really like to see Zygarde in the mod, and it could be handled quite similarly to Sun and Moon without being too much hassle to implement afaik, just look up how you get Zygarde and forms in Sun and Moon if you want clarification.