Chad: I believe the answer is "conformed to the likeness of his Son" via
"the sufferings of this present time" (vs. 18).
JGR:Where does "sanctification" fit into these golden links? Rom 8:29-30 29
For those God foreknew he also predestined to be conformed to the likeness
of his Son, that he might be the firstborn among many brothers. 30 And those
he predestined, he also called; those he called, he also justified; those he
justified, he also glorified.

Carlo:  It's possible that the "conforming to the likeness of His Son" can
be sanctification.  I stress "possible" and "can." It's also possible that
the text above doesn't outline sanctification.   Sanctification is defined,
I believe, as a work of God's grace by which our whole person is made new in
the image of God, and we are able to be made more and more able to become
dead to the power of sin and made more alive to righteousness.  One might
reason that the conforming to the likeness of Jesus, could be sanctification
in the text.  But I don't think it's clear.  The conforming could also be
the order of salvation of the "Union with Christ."  Either way, more clear
texts that speak of sanctification are Rom 6:7; 1 Cor 6:11; 1 Pet 4:1 - for
definitive sanctification.  For progressive sanctification you have 2Thess
2:13; Rom 8:13; 2Cor3:18; 7:1.

Notice some other ordo salutis elements at least explicitly or clearly
missing from Rom 8:29-30 - (1) regeneration; (2) conversion (faith and
repentance); (3)perseverance/preservation of the saints.

What's important, in my view, in order for sanctification to take place,
there actually has to be a person (born), in order for the facets of
sanctification (definitive and progessive) to take place.  Sanctification
does not take place without the person.  My body did not exist in eternity
past when I was elected.  Nor was there any conversion on my part in
eternity past.  I did not exist.  I was not unioned with Christ when my
election was made.  The Lord did not declare me not guilty at eternity or
nor was I glorified and nor will I be glorified with a new body until Christ
returns.