Grace seems a plausible suspect because:

There was a hint (I think in series 2) that she had a traumatic past, but we have not found out much else other than that she really gets touchy on the subject of suicide (she snaps at Rigsby when he makes light of a woman who has apparently jumped of a bridge to her death).

Also, why have we been given more insight and back -story into the other 'central'  CBI agents (Theresa, Cho and Rigsby)? For example we learn about Theresa's closeness to her brothers and the affect her alcoholic father had on her life; we meet Rigsby's father and learn about his criminal background; Cho has an entire episode dedicated to his ex- gang lifestyle, so why is Van Pelt's background information limited to the fact that we know her dad was a competitive football coach and she seems to have lost her sister?

She has red hair and as we know women with red hair often appear in episodes. She was the last person to join the CBI team, and I think this was revealed to be not long before Patrick Jane joins the CBI, in the episode which flashes back to when he joined the team? 

She is also reluctant to transfer to another department when her relationship with Rigsby is made public, and is particularly keen to keep their relationship under wraps in the first place- this may seem sensible because she was last to join and a woman keen not be overlooked- but could it also indicate that she needs to stay close to Jane? 

During the most recent season, has the actress been missing due to pregnancy? If so that conveniently explains why Grace was written out to go and complete a computer course to develop her skills and career, but this would also be a convenient amount of time to go elsewhere and develop/do Red John tasks!

Grace is a whiz on the computer, and is often the character who encourages Jane's tricks- much like Red John, who enjoys keeping track on Patrick's activities and seems to me to respect and admire what he does.

I think it would be fantastic in the final episode to go back in time to each key episode and for it to be revealed that Grace's word or actions have had a sinister double meaning/ulterior motive! This would be so dramatic and would really be a terrific example of writing! Unfortunately I don't know how likely this is going to be....

To me, the writers of 'The Mentalist' wish to succeed in one task, when Red John's identity is revealed - they wish to cause ripples of shock within the audience. Take a look at 'Broadchurch', the killer was someone close to the detectives and was someone whom went under the radar during the investigation. This in 'The Mentalist' terms would be someone who is on the team, Grace is the most likeliest Now I am a few weeks behind than the US so I don't know about the last two episode of this current season. The first episode (If I remember correctly) she is starts at the same time as Patrick and shakes hands with him (that's the clue given by Lorelei). This creates a certain connection to him and if she is/a connection to Red John than she is able to keep up on the investigations of her/or person in connection killings. She often talks about her Father, a football coach, now her father would be the stereotypical age that Red John would be so perhaps he is and she is in connection as 'watchdog'. She has a trauma  in her past which we know nothing about- so this adds a mysterious dynamic to her. She often challenges Patrick's psychologically 'tricks' which highlights the difference between them. If I remember correctly, she had a fiancee who he ended being something to do with Red John. I can't really remember about him but perhaps he did not know about Red John's true identity. She recently got back together with Wayne, this creates even more shock value as it highlights and reminds the audience of their relationship and will create more shock wave ripples. Also her name, it seems forced to me. Grace Van Pelt. The Van Pelt part as the same letters as Red John, the writers don't always choose a random name, the Van Pelt part seems deliberate and forced to imply a connection. So,all in all, she has something to hide, the writers have 'red herrings' (ie. to divert the attention of the audience) so for example that creepy FBI investigator  they are all 'red herrings' and the true Red John is secretly lurking in the CBI department. She could easily be Red John or is acting as 'watchdog' for her secret football coach Father...

Van Pelt is ginger and unlikely to have a soul. Case closed.

I think there is a link between Red John and Van Pelt.

What if Van Pelt were actually the real main character of the Mentalist ? Indeed, the pilot episode deals with Van Pelt first coming to the CBI, and I'm convinced the first clues are given in this episode : she may be the key to solve the Red John mystery. Maybe the fact she is red-haired is a sign to the viewers ? Let me explain you my theory, or more precisely some interesting elements about Van Pelt I noticed.

First, she always has troubles : her strange boyfriend who is actually the killer the team was looking for, Craig O'Laughlin who is a RJ's associate, ... It looks like if she had a very bad luck. And, there is this trauma : "... you're deeply repressed and emotionally shut down... because of a trauma in your past that you've never spoken of to anyone... ever... even yourself..." that Patrick talks about in the first episode... Maybe the suicide of her sister ? A very fuzzy situation here, and we never had any answer to this question.

Moreover, she appears in almost every episode of the show and she is the one who seems the most interested in Patrick's tricks. Grace believes in High-beyond, with médiums, etc,... Van Pelt says in the dinner scene (in the pilot) that her aunt, Yolanda,  was a "real" psychic (what Janes refuses to admit). And in an episode with the leader of Visualize, she seems to appreciate the advice given by Brett Stiles.

Also, she is also specialized in data processing, and Red John proved that he was also very skilled in that domain. He intruded the CBI computers at the moment Van Pelt was on hers.

She smiles at the end of an episode when she sees PJ crying.

In the last episode of season 2, in which PJ meets RJ, RJ ends telling Patrick : "Roll Tides", which could be linked with a football, and her father, Amos Van Pelt, were trainer of football. And Craig O'Laughlin knows VP's father as he trained him at school.

To sum things up : she is red-haired, she shows up in the first episode, she has odd connections with some of the murders, she is skilful with computers, she has connections with magic and mentalism, she never talked about her trauma, her father is frequently evoked but we don't know who he is... And more !

My guess ? Red John could be Van Pelt's father or somebody in VP's family (maybe Yolanda ?). What if Van Pelt were Red John ?

This is more of a question and a thinker than a theory.
The question part is, is Van Pelt's father a football coach?
Thinker part is "He is man", Heisman trophy, football, you see where this is going...

Think at that.... why know red john fron rosalinda description ... but i think rosalinda is van pelt sister because the look very similari... we know she has a sister that have a trauma and she didn t speak about her .... kristina frye escape when van pelt was with her !!!!!!  here i think is van pelt voice : http://www.youtube.com/watch?v=ns_DWnqrEdI ...... she was living on a farm !!!! that 2 murders that red john make at the farm !!!! every one is watching for a man ... but RedJohn is a woman ... that are the things that comes now in my mind ... but i think it s more ... 

(This is NOT a theory of RJ)

In S05E05 Red Dawn, when Hannigan moved to another unit, there was a quick scene with the profile of GVP. I tried getting the text out of her bio, so here's what I have (some words are unknown, referred to as ???? because the text was so small to read):

"Van Pelt is from a small farming town in Iowa. She tends to be overly sensitive at times, but is very intelligent. Van Pelt works with Det. Chillings most of the time, but occasionally works with the FBI or SFPD. She is the gold standard of the team, having started her job in the year 2000. Grace's cousin is supposedly a psychic, causing her to sometimes argue about matters of faith. In the case of 788728-009, the suspect made a remark about Van Pelt's character, stating "...you're deeply depressed and emotionally shut down...because of a trauma in your past that you've never spoken with anyone...ever...even yourself..." which Van Pelt's facial expression seems to confirm. In the case 459959-001 she is ???? for ???? due to a killer who is killing red haired people, leading her to being in an uncomfortable situation"

- The psychic was her cousin Yolanda
- The trauma referred to is probably Van Pelt's sister which she lied about to Rigsby (S02E10 Throwing Fire)
- The serial killer part is very interesting