The Lorax was one of my favorite movies as a kid (in all of its chaotic glory), and we just rewatched it for the first time in a while. And I had had this thought a while back but was today reminded of it. . .but there are a lot of similarities between Lorax characters and Gravity Falls ones.

To start off with, the one I mentioned on my message board just a bit ago:

Gideon as Aloysius O'Hare.

As mentioned on my message board, they're both antagonists who are short, wear blue suits, and have ridiculous haircuts.

Dipper as Ted Wiggins

As far as personalities go, these two aren't super similar, but they're both brunet main characters with crushes on an older red-headed girl, which leads to the next one:

Wendy as Audrey

I'm not sure why Wendy would really really want a Truffula tree (it's kinda ironic considering her dad is a lumberjack), but we're just gonna roll with it.

Stan and Ford as the Onceler and Lorax

I think it would be Stan as the Onceler and Ford as the Lorax, but I could also the roles being reversed. It makes sense for Stan to be the one still there, talking to Ted/Dipper, while Ford is off wherever the Lorax disappears to; on the flip side, though, I could see Ford inventing a Thneed and being so so determined to become famous with it. And Stan and the Lorax are just very similar.

Either way, it's two grumpy old guys who used to be friends, had a fight and didn't see each other for years, but were eventually reuinted.

Soos as Cy (the delivery guy)

This fellow right here. He and Soos have a very similar energy

Bud Gleeful as one of these guys

They carry O'Hare sometimes, they do whatever he says. . .sounds pretty similar to Bud to me.

Mabel as . . .?

My original thought was that Mabel would split the role of Ted with Dipper; I'm sure she'd be 100% down to help her brother find a Truffula tree for Wendy, and she'd probably love being able to use some of the tufts for her knitting.

But then I was talking to my siblings about this crossover and they suggested Mabel being Grammy Norma. . .

. . . which actually fits really well?? Especially because I wasn't sure who would be Ted's grandma or mom in this (although there was a vague idea of Stan maybe being the grandma but idk how that would work if he's the Onceler or Lorax)

So anyways, there's that whole thing XD 
I think it's a pretty fun crossover idea, and some of the parallels between these two things are pretty funny.