As some of you may know Delazar78 and myslef are currently in the process of converting ships and monsters from the Spelljammer 2E campaign setting to5E rules.
I have also been throwing around some ideas in my head as one of my players has been bugging m,e for almost a year to have a flying airship so I might just turn it into a Spelljammer ship. My PCs are currently level 6 and I thought I would let them dip their toe into the Spelljammer universe with things like Neogi/Scro raiding the lands near where they are.
The grand final would be a supwerweapon more or less ripped straight from Star Wars as the Beholders have built the Death Eye which is a massive (500miles wide) asteroid that can emit beholder eye beams on a planetary scale. A bright light can appear in the skies at night getting bigger and bigger until half the PCs home planet gets bathed in an anti magic cone. The Death Eye may not be fully operational but the Beholders would be building up to a disintegrate ray on the whole planet.
The DMG also has a few things which make a SJ conversion easy. Siege weapons and renaissance era weapons (pg 255 & 278)
Option B is the Death Hamster a Gnomish superweapon powered by giant hamsters which blows itself up.
Nothing is set in stone anyway so does anyone here have some idea. Even if you do not have that much knowledge of Spelljammer it is a kitchen sink setting so you could have things like a planet of Dragonborn or whatever in the setting.
I like more the Death Eye idea... I think Spelljammer is silly enough without adding even more silliness... 
I would have one of the ships from the good guys crash near the PCs base of operations. The crew needs materials to fix their ship, and while they visit the PC world, they get friendly with the Heroes, eventually asking them for help.
Or maybe the visitors from the void came looking forthe PCs, since one of them (or all of them) is the One that can help them stop the beholders. Reason being that they have illithid blood in their veins, and they will be able to interact/activate the beholder weapons.