Was Tracy Mills a part of John Doe's plan from the beginning?

Question

In Se7en, John Doe is shown as being extremely meticulous and patient while carrying out his plan of murdering and/or torturing people in accordance with the seven deadly sins.

At the end of the film, it is revealed that John Doe had killed and decapitated Tracy Mills, the wife of Detective David Mills.

Was Tracy Mills his intended target from the beginning? Did John Doe decide to kill Tracy only after Detective Mills and Somerset discovered his residence?

Answer 1

Was Tracy Mills his intended target from the beginning?

No.

The Mills have only just moved to the city...

In an unnamed American city, soon-to-be-retiring detective William Somerset is partnered with short-tempered but idealistic David Mills, who recently transferred to the department, moving to the city with his wife Tracy.

^Wikipedia

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                             MILLS
              I'm a little thrown.  I just got in town
              like twenty minutes ago and they dumped me
              here.

^Script

Whereas the killer has been carrying out his plans for weeks if not months. Victor, the starvation victim, had been there for A YEAR.

The female cop has gone to one wall where a sheet is pinned up.
She pulls the sheet down.  Pinned behind the sheet are **fifty-two**
Polaroid pictures; all pictures of Victor tied to the bed, with a
date written at the bottom of each picture.  It is a visual
history of Victor's physical decay.

..snip

                             MILLS
              All pictures of Victor tied to the bed.
                      (crouches, points)
              The last one is dated three days ago.

Somerset looks at the first photo.  In it, Victor is bound and
gagged, but he is healthy.

                             SOMERSET
              The first one... it's dated one year ago.
              To the day.

It seems highly unlikely that the Mills' would have been included in those plans if they weren't even in the city or involved in the case until now.

Indeed, if John Doe was targetting the Mills' from the start, he would know where they lived.

Instead he had to discover this information

                             JOHN DOE
              It's surprising how easily a member of the
              press can purchase information from the men
              in your precinct.

Answer 2

Was Tracy Mills his intended target from the beginning?

Highly unlikely.

We know the killer has been working on this for months. We don't know whether he changed his plan after meeting Mills. This leaves us 2 options:

• He had targets for all 7 sins, but was forced to change them due to Mills

• He had decided to use the cops assigned to his case (with some luck, they could be triggered into the final sins)

In the first scenario, Tracy wasn't part of the plan, as wasn't Mills. He just had his 7 targets, and was going to execute them. When his hand was forced, he decided to use Mills and his wife. In the second scenario, he has to wait for Mills to be assigned, and again, Tracy is an unknown factor, as the cops assigned might not have had wives, and instead children or something.

Did John Doe decide to kill Tracy only after Detective Mills and Somerset discovered his residence?

Essentially, yes. Although we don't know if he had decided to use the cop and someone he loved from the beginning, or just some specific targets for those sins. But as they forced his hand, all we see is his final plan.

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This answer presents the same theory.