No, not that "Dating"
I realize my title may be a bit ambiguous, this thread has nothing to do with Caracalla's love life, or Plautilla and her relationship with Caracalla, a sad story for which there are other sources e.g. this book.
What date for this coin?
Instead, this post is focused on trying to decide - "could I narrow the date range for my coin". I recently took a closer look at a Roman provincial coin from Augusta Traiana and wonder how close to the death of Septimius was the minting of this coin? Augusta Traiana was located in Thrace (near the modern town of Stara Zagora, Bulgaria).
Sequence of Portraits for Caracalla
After picking a few representative portrait types for Augusta Traiana, I found this 2014 study, written in German, with beautiful pictures of coins and a detailed assessment of the Roman denarii portraits from Andreas Pangerl, "Roman Imperial Portrait Types of Caracalla and Geta". I used the portraits from this article as my timeline. Here is the rough progression of portraits from Augusta Traiana that I assembled with the dated Roman mint coins aligned.
Here's the summary of how Pangerl classifies the portraits of Geta and Caracalla:
with short hair (Caracalla Type 1, Geta Type 1)
with curly hair (Caracalla Type 2)
Adolescents to adults with short hair
no beard (Caracalla Type 3, Geta Type 2)
with sideburns (Caracalla Type 4, Geta Type 3)
with cheekbones without a mustache (Caracalla Type 5, Geta Type 4)
with cheekbones and mustache (Caracalla Type 6, Geta Type 5)
short full beard (Geta Type 6)
with long full beard and fringes (only Geta type 7 looking a lot like his father)
short full beard + laurel wreath begins on coins - starts in 209, probably with get raised to Augustus, youthful and emotionless (Caracalla Type 7)
short full beard, deep forehead wrinkles, "wild" facial expression starting 211 -(Caracalla type 8)
short full beard, less pronounced forehead wrinkles – starting 213-214 (Caracalla Type 9)
He also considers another type for Caracalla looking "exhausted or resigned" starting in 217, but considers it too close to Type 9 to separate. There is an interesting suggestion in the paper: Geta's portraits may suggest the Geta was attempting to overthrow his brother, portraying himself as "Septimius-like".
My provincial portrait seems to align well with portrait type 7 for Caracalla which becomes increasingly difficult to distinguish from portrait type 6 of Geta as political messaging focused on secure succession with two Augusti, i.e. AD 209-211.
There is an Augusta Traiana coin of Geta, as Augustus, with a portrait very similar to the one of Caracalla on my coin. Which I will link to discussion from Pangerl, "At the death of Severus in February 211, the portraits of the two brothers can hardly be distinguished from one another", politically supporting the decision that the two would be together successors.
Aligned with the portraits, the sharing of the globe on the reverse, seems to suggest the time when both were Augusti, if not the period when they co-ruled after Septimius.
Here is the coin of Caracalla as Augustus with the two loving brothers sharing a globe. This seems to be a very rare coin - only one possibly two others that I have found of this type other than this coin both with matching dies (one in ACSearch). The exact coin not listed in Varbanov, however Corpus Nummorum has a coin that also looks like it might be the same dies (CN 9717).
Thrace, Augusta Traiana, Caracalla (198-217)
Obv: AVT K M AVPH ANTΩNINOC, laureate bust right, with slight drapery.
Rev: AVΓOVCTHC TPAIANHC, Caracalla and Geta standing facing one another, both togate holding globe
Ref: Varbanov 1092 (unlisted variant, no globe), CN 9817 (same dies?)
Note: Could the globe be a die break? Possibly, I was not able to find a die match without the globe, without more evidence, I can't be certain one way or another. Unrelated, I remain skeptical of the idea that this coin shows Caracalla stealing the basketball from Geta.
With all of this, I have reasonable confidence that I can narrow this coin's date to the time period between Geta rising to Augustus (end of AD 209) and Geta's death (December, AD 211), and perhaps even a bit closer to the death of Septimius Severus.