I'm baffled at how much they screwed it up. They really didn't need to change the formula much for the sequel to be a success. Just come up with a new portal or some excuse why the portal from movie 1 didn't close and do the same thing as the first movie.
Instead they did whatever they ended up doing in movie 2 and killed what could've been a fun blockbuster franchise.
Hell they could've adapted one of the the hundreds of fanfics online and it would've been a better starting point than whatever they ended up with.
I look at it a different way - it's a miracle the first one was good. If you imagine hearing the premise for the first time, it sounds like a trashy movie with no depth. Somehow it ends up being way more than that, and I'd suggest that's because of excellent direction. Without that it reverts back to trash.