So, because I am intrigued (and because I am trying to avoid housework) I thought it would be fun to make a list of canon characters that haven't been mentioned, and characters that have been mentioned but don't seem to be using journals, building off of what other fen, especially
lapin_agile have noticed.
Students:
Justin Finch-Fletchley (not mentioned--Muggleborn)
Anthony Goldstein (not mentioned--Muggleborn?)
Parvati Patil (not mentioned, but Lucius makes reference to gov't business w/someone named Patil)
Padma Patil (same as Parvati)
Lavender Brown (not mentioned, but Lucius makes reference to a Mrs. Brown who might be her mum)
Cormac McLaggan (not mentioned)
Ernie MacMillan (I believe there was a brief reference to his family by Lucius)
Cho Chang (mentioned on Quidditch rosters)
Cedric Diggory (mentioned on Quidditch rosters)
Vincent Crabbe (mentioned by various Slytherins, including an unfortunate eybrow incident)
Gregory Goyle (mentioned by various Slytherins)
Oliver Wood (mentioned as the student who observed Quirrell)
Lee Jordan (mentioned briefly by McGonagall?)
Katie Bell (mentioned on Quidditch roster)
Angelina Johnson (not mentioned i don't think)
Dean Thomas (not mentioned--Muggleborn)
Seamus Finnegan (mentioned by Ron and Neville. Is it is known that he's a half-blood?)
Marcus Flint (mentioned on Quidditch rosters)
Zacharias Smith (mentioned by Harry, who still doesn't like him)
Daphne Greengrass (annoyed mentions by Slytherin girls)
Colin Creevey (Muggleborn--mentioned by Order members as being at their sanctuary)
Eloise Midgen (mentioned by Hannah)
Susan Bones (mentioned by Hannah)
Faculty/Staff:
SNAPE! (has not been mentioned by anyone, which is intriguing.
alt_player has informed us that there will definitely be a Snape!)
Flitwick (not mentioned--Charms is taught by an OC)
Hagrid (almost mentioned by Sirius--apparently has S.'s motorbike)
Filch (Squib, so probably not working at Hogwarts anymore--mentioned by Sirius as having done so in the past)
Sprout (has been mentioned several times)
Trelawney (mentioned by Molly--is apparently in America with Dumbledore)
Carrows (mentioned but--fortunately--don't have journals)
Quirrell (has been sighted around the school behaving strangely, apparently taught Muggle Studies and was sacked for being too Muggle-friendly, according to Lucius
Slughorn (is teaching Potions and apparently well-liked by student; no journal though)
Lockhart (is teaching DADA which he didn't do until 2nd year in canon, mentioned by several students)
Sinistra (mentioned--students apparently enjoy staying up late for astronomy)
Vector (mentioned)
MacNair (a Death Eater in canon, here he is head of Gryffindor and teaches COMC)
Others:
Peter Pettigrew (has not been mentioned by anyone, as
lapin_agile pointed out, although I believe Sirius made a veiled reference to a previous betrayal when discussing his doubts about Remus
Andromeda Tonks (mentioned by Sirius, as well as Narcissa and Bella. Current status unclear)
Ted Tonks (not mentioned, except maybe indirectly by Sirius)
Nymphadora Tonks (not mentioned, but there is a journal set up for her that hasn't been used yet)
Bill Weasley (mentioned as the creator of the Order Only journals)
Charlie Weasley (not mentioned)
Fleur Delacour (not mentioned, but it probably wouldn't make sense for her character to appear this early anyway)
Viktor Krum (see Fleur--and boy do I ever wonder how they'll handle his connection with Hermione in this game)
Rufus Scrimgeour (mentioned as head of MLE, Lucius dislikes his insistence on constitutional protections)
Delores Umbridge (appears to be Minister of Magic)
Amelia Bones (not mentioned)
Dursleys (not mentioned--but possibly alluded to by Harry when he mentions having met Muggles?)
Alastor Moody (not mentioned)
Mrs. Black and Kreacher (mentioned by Narcissa)
Rodolphus Lestrange (mentioned by Bella, who clearly bosses him)
Rabastan Lestrange (not mentioned)
Kingsley Shacklebolt (mentioned by Order members--is a clever but beleaguered agent)
Frank Longbottom (mentioned--is smuggling muggleborn babies with Alice)
Rita Skeeter (mentioned--Lucius is forcing her to write for the Quibbler)
Xenophilius Lovegood (has been discussed by the Order, is currently living with the Weasleys)
Aberforth Dumbledore (not mentioned)
Various canon aurors and death eaters (now respected government officials) have been mentioned.