The problems that arise from the mixing of the worlds? no clue really as Lego Movie 2 barely had any meaning compared to number 1.

OR they finally introduce Bionicle

Here's my pitch:

In The LEGO Movie 3: The Final Step, Emmet and Lucy leave their world to start a new life as Finn leaves for college, leaving Sweet Mayhem as the new protagonist who wonders what to do in their absence. She gets summoned by a guardian of dimensions (represented by a new friend of Bianca's with a huge LEGO collection) who seeks to unite all the themes to stop a character from a theme that was rejected by the LEGO company from destroying the company and "cancelling" all the themes.

So it's kinda like Toy Story 3 meets Spider-Verse meets LEGO Dimensions.

Kinda like No Way Home

This could even be like Kung Fu Panda 4 with old characters returning

Emmet gets captured LOL

Emmet and Lucy would not appear in Lego Movie 3 because Universal owns the rights to the franchise

What? They did?!

Yes after the last two movies failed, Warner Bros gave Universal the rights

Failed? They failed? Their like my favorite movies!

I mean box office wise

Oh. Well I guess no more Emmet and lucy.