Taylor Swift Fans Think She'll Be Lana Del Rey's Surprise Coachella Guest

Taylor Swift's loyal supporters think they've got a new theory about her all figured out -- and based on the clues, they might be on to something ... namely, a possible Coachella drop-in.

Here's the deal ... with Tay Tay announcing her new album last night during the Grammys -- "The Tortured Poets Department" -- Swifties have been kicked into high gear with speculation that her release date, April 19, is no coincidence ... especially considering who she came onstage with later in the evening during her Album of the Year win.

Of course, we're talking about Lana Del Rey ... who was among a small group of people who joined T-Swift at the end of the show when she accepted her big award.

Taylor insisted on dragging her up there ... not to mention pouring on a ton of praise -- calling Lana an absolute legend and giving her her flowers and then some in her speech.

It's because of this strong friendship that people are thinking Taylor might hop onstage during Lana's headlining set at Coachella in April -- it's well-known at this point, the moody crooner is going to handle Friday night duties, including a big Weekend 2 set on Fri. April 19.

What a coinkidink ... that also just happens to be the day Taylor's dropping her project!

Some are suggesting Taylor's gonna take the opportunity to make a big splash to get people to listen -- which she could most certainly do without Coachella ... but something like that would absolutely break the Internet, and that's exactly what many believe will happen.

If Taylor does hit the main stage at Coachella ... it'll be her first time there, as she's never headlined before -- and she hasn't made a guest appearance either in all these years.

Lop on the fact that this year's lineup is getting crapped on left and right for being underwhelming ... and you can see why a Taylor cameo is just what Goldenvoice needs.

Oh, and did we mention that she has this April window open??? Yeah, she'll be on a break from her international "Eras" tour between March and May ... just in time for something like this. We're not saying we've heard one way or another, but fans sure seemed convinced.

If this does come to pass as many suspect it will ... it might be one of the biggest Taylor Swift moments in a long time, and that's saying something considering the years she's had.