Although we simply are not told what Jesus "wrote" in this action, there has been informed (?) speculation about it for centuries. We need first to make two quick observations on the Greek:

• the very fact that the account has Jesus writing on the "ground" (γῆ = gē) precludes the possibility that he is writing directly on to stone (as already pointed out in a couple related previous Q&As);
• it's worth noting, too, that the verb in verse 6 is κατέγραφεν (kategraphen) which can mean to "write", but can also mean to "draw", as recent commentators often point out. That is, Jesus might just have been doodling. (Yes, simple graphō is used in v. 8.)

Since the time there was commentary on this text, there has been speculation about what Jesus wrote, and why. That interest has continued unabated over the centuries, and past decades of scholarly writing are just the same.

There is something that connects this episode, now residing in John 8, with the Decalogue -- an allusion noticed as early as Ambrose, the fourth century bishop of Milan, and has been argued for as recently as 1990, in J.A. Sander's contribution, "'Nor Do I...': A Canonical Reading of the Challenge to Jesus in John 8," in The Conversation Continues: Studies in John and Paul, ed. by R. T. Fortna and B. R. Gaventa (Abingdon, 1990) pp. 337-47.¹

But this interesting suggestion takes its place beside the welter of other suggestions accounting for Jesus' actions in this pericope. The most complete and conveniently available catalogue of suggestions (which still stops short of being absolutely comprehensive) is in Raymond Brown's Anchor Bible Commentary.² Here's the quick run down:

• Starting with Jerome, there is a suggestion that Jesus wrote the sins of the accusers.
• T.W. Manson, in a widely cited article: "The Pericope de Adultera (Joh 7₅₃–8₁₁)", Zeitschrift für die neutestamentliche Wissenschaft 44 (1953): 255-6, argued that Jesus' actions reflected Roman legal practice: writing the sentence (8:6), then delivered (8:7), and wrote again (8:8) what he would say in v. 11.
• Some find echoes not of Exodus/Deuteronomy, but of Jeremiah 17:13 which speaks of "writing on the earth".
• J.D.M. Derrett proposed (1963) specific connections to Exodus 23:1b, concerning the prohibition against being a malicious witness (as this mob were).
• Some suggest (with a bit of psychologizing/narrative-gap-filling), that Jesus is just biding his time, with various grounds suggested. (Not mentioned by Brown, but this is very close to John Calvin's interpretation.)

As noted, this is not a comprehensive list: do see the Google Scholar link to explore many other proposals and arguments, should that be of interest.

Notes

1. See the account in Chris Keith's important study, The Pericope Adulterae, the Gospel of John, and the Literacy of Jesus (Brill, 2009), p. 12. He follows up this suggestion later in the book.
2. R.E. Brown, The Gospel According to John I-XII (Yale Anchor Bible 29; Doubleday, 1966), pp. 333-4; see his commentary for further details.