Do not continue if you haven't seen season 3 final episode yet. Also don't read if you are not a major fan and not in to deduction. As a big fan of intelligent written and filmed TV shows like Sherlock i've come to the following deduction
Please read till the end :
I will bring it right to the point :
-Magnussen is still alive
-Moriarty is still alive (at least the REAL Moriarty)
Magnussen has a device in his eye to project the information on his glasses, the writers always used new technologies in this show so why not google glass etc..A year forward or more for Season 4 i will be more likely.
I will proof this through points and pictures below.
Proof :
-The on screen text when CAM is searching is always bended text like reading from the eyeball.
http://imgur.com/RNIH0sc
http://imgur.com/eJ0epHM
http://imgur.com/odKevo1
http://imgur.com/TZwfPVv
http://imgur.com/OGGunrj
-Moriarty is still a pressure point for Sherlock ? Why.. because Sherlock knows the REAL Moriarty is still alive
http://imgur.com/odKevo1
-In his 2 years abroad Sherlock probably found out that the REAL Moriarty is still out there
-CAM is the perfect person to have a pressure point on Moriarty which Sherlock now knows he needs to defeat him
-CAM is the perfect person who has acces to all media to display a fake Moriarty on all the channels to smoke the REAL Moriarty out the and to send Sherlock back at the exact moment he took off.
-Sherlock figured out how Moriarty faked his death and used this technique on CAM
The eye theme proof :
-In the first act CAM sits in a questioning, at one moment the shots is from above and eye decorations are seen on the floor including eye lens schematics.
http://imgur.com/6ZniFgl
-Sherlocks mentions the dead eye stare multiple times
http://imgur.com/C1e5XhC
http://imgur.com/YLrO4rj
http://imgur.com/Dmg0iiJ
-Sherlock mentions the camera at eye height at the elevator, but as a tall man he has to look up to the camera
http://imgur.com/ZVslYjg
-CAM picks out an olive (eye) on Sherlocks plate
http://imgur.com/6UKyYum
-CAM has an eye symbol next to his “vaults”
http://imgur.com/IYBCCli
-When Sherlock and Watson walk to the vault the camera moves to a lens like object/vase
http://imgur.com/Vb53Qov
-CAM in itself is reference to Camera ?
-CAM is never searching without his glasses/projection screen on
Also the actor that plays Magnussen is carefully introduced and is of a to good a quality to just use for one episode. In the beginning of episode 3 it is also revealed that Magnussen has a medical issue of always having sweaty palms wich is never revered to again...yet. Elisabeth Smallwood is also has a bit to much screen time not be seen again… she is the one who decided with Mycroft that Sherlocks needs to come back.
I think Sherlock got the pressure point of Magnussen figured out, realized through some events from the end of season 2 throught the ending of Season 3 that Moriarty or another evil (brother of not revealed yet) is still lurking and decided it was a bigger threat than Magnussen and used his media ability to show Moriarty just as he was send off. It could also make sense he figured out the technique Moriarty used to shoot himself and used it to "kill" Magnussen.