First topic of intrest. As we all may of noticed in the most recent chapter, his mask.
A possible indicator of Ichigos growth? Or possible indicator of control over his hollow increasing? Both, his hollow gives a bit of advice before being completely suppressed.
Bleach 221 page 17 | One Manga
As stated he's allowing Ichigo to use his power at will, and over time his ability to use it more efficiently is increased over time. Or it coul also be intrepreted as him lending his power on a gradual rate, until he masters it with one final battle. It seems his hollow spelled it out for us awhile ago and below is the evidence.
Here is the birth of Ichigos hollow, and Shinigami abilities. While his power is still new and unrefinded, two lines are visable at the top, and unconnected.
Bleach 63 page 17 | One Manga Bleach 64 page 12-13 | One Manga
Now here we see our first noticable change. After his fight with Renji, his mask developed another stripe, still no connection.
Bleach 100 page 04 | One Manga
The next change is noticable after his battle with Ziraki. His mask takes on another stripe under the eye. Lines on the top are closer to being connected.
Bleach 116 page 18 | One Manga
The most dramatic change was after he won his inner struggle against his hollow. Resulting in another stripe under the eye, and three more under the bottom row of teeth, all connected.
Bleach 222 page 10 | One Manga Bleach 222 page 14 | One Manga
After leaving his training to protect his friends he shows up and finds Grimmjow looking for him. Not much time to increase his power, no changes.
Bleach 231 page 13 | One Manga
Same goes for his first encounter in HM, no change.
Bleach 253 page 14 | One Manga
His third encounter against Grimmjow following being beat by Ulquiorra. He gains one more under his eye, and his lines are much more thick, and undefined. Maybe hinting at a molting state before a exponential boost in power?
Bleach 281 page 19 | One Manga Bleach 283 page 03 | One Manga
The last and most current mask. The mouth seperates becoming two seperate parts, unlike any before. The lines are more defined when compared to their thickness in the Grimmjow battle.
Bleach 344 page 10-11 | One Manga Bleach 344 page 13 | One Manga
To conclude they indicate both control over his hollow, and growth in power.