In Jn. 20:28, Thomas possibly calls Jesus Lord
and God while not equating him with YHWH (Ps. 8:5-6;
82:1-6). Jesus had already been identified as "a god"
(NWT) in Jn. 1:1 and as "the only-begotten god" (See
N.T. Wright's tentative translation of this passage in
his NTPG) in Jn. 1:18. But John equally made it clear
that men could be called "gods" without transgressing
the boundaries of Jewish monotheism (Jn. 10:34-36).
The Lord also spoke of "the only true God" in Jn. 17:3.
And Jesus did not identify himself with "the only
true God" (See Raymond Brown in his Anchor Bible
commentary on John).
I thus believe that whether one decides biblical verses
such as Jn. 20:28 are subject nominatives or
nominatives of address, he or she can still hold that
Jesus is not Almighty God. I make these preliminary
statements in view of what might be an example of the
nominative for a vocative, namely, Jn. 17:25:
Πατὴρ δίκαιε, καὶ ὁ κόσμος σε οὐκ ἔγνω, ἐγὼ δέ σε ἔγνων, καὶ οὗτοι ἔγνωσαν ὅτι σύ με ἀπέστειλας