After watching "Rarity Investigates" and re-watching "Where on Earth is Carmen Sandiego?" (the cartoon), I got thinking how it would be possible to crossover/fuse the two...
The most obvious would be a direct "Carmen Sandiego in Equestria", but maybe there is too more interesting possibilites:

1) News about a "Carmen Sandiego"-like criminal began to spread in Equestria and the Mane6 is called to investigate and capture the criminal. It would allow the chance to see a Twilight Sparkle in "Sherlock Hooves" mode and Rarity in "Shadow Spade" mode.

2) In an alternate universe, maybe one of the Mane6 is the Carmen Sandiego-like thief. One possibility would be Rarity since she loves gems and she can pull the CS look (as the image below shows )

This another fanart shows an very intriguing possibility - in special for a Dating Catwoman scenario (check the jewel holding her mane)

What do you think?

5086009 Or to add to your second one.... all of the Mane 6 are Carmen Sandiego-like thieves. They just change disguises to throw everyone off.

I like the idea of a Carmen Sandiego-esque thief. Maybe named Crime O'Century?

5086027 Oh, like a criminal version of Mare-Do-Well? Caramel Marediego?

5086041 That works..... SOMEONE MAKE A STORY DESCRIPTION OF THIS NOW!

I'm still waiting for someone to ponify this song:

5086027

Or to add to your second one.... all of the Mane 6 are Carmen Sandiego-like thieves. They just change disguises to throw everyone off.

5086041

Oh, like a criminal version of Mare-Do-Well? Caramel Marediego?

It could work too, through the idea is closer to Mare-Do-Well than cartoon version of Carmen Sandiego...
But I think that it would be interesting if the other members of Mane6 aren't aware that one of them is a master thief - and the master thief one using trick after trick to prevent the discovery.

5086036

I like the idea of a Carmen Sandiego-esque thief. Maybe named Crime O'Century?

Crime O'Century?? That is a pony name so obvious that it becomes brillant! Great idea!
I confess that I was playing with names like "Scarlet Ghost" or "Phantom Ruby"...

Other possibilities:
3) (Based on the "Power Ponies" episode) One of the Mane6 buys a book of adventures of a master thief, but it was enchanted as Spike's comic. Now she (I'm thinking Rarity or Twilight) have to stop the book's villain protagonist's crime spree to return to real world.
4) In a inverse route, a magic student (Not Twilight - let give her a break) enchants a book but it misfires, and it brings the master thief to Equestria - and now the Mane6 and the Royal Guard have to capture her...

5086070 Scarlet Ghost and Phantom Ruby work too.

5086070 Oh look, I found Waldo too!

5086009 Well, here's another idea, if you're interested:

The actual "Where on Earth is Carmen Sandiego" version of Carmen uses some of her old contacts to break into A.C.M.E. headquarters with the intent to hijack the C5 drive (much like she did in an actual episode one time), but shenanigans occur to send her, some of her V.I.L.E. henchmen, and of course Zack and Ivy to Equestria.

You could even probably arrange it in the vein of the current Player is also a Brony, and had, like, an episode from the show open in another window on his computer or something. In that case, you could have some interesting interactions between Pinkie Pie and the gumshoe siblings, being that the former leans so heavily on the Fourth Wall without truly breaking it for those around her, and the latter actively converse and make requests of the Player THROUGH the Fourth Wall.

Plus, how could we not put Discord and C.H.I.E.F. in the same room together?

Wow!

5086274 Discord and C.H.I.E.F. in the same room together? I think their combined sarcasm would make a black hole and suck in everything.

Where is that Dating Catwoman pic from?

Gumshoes! Crime O'Century just stole the Applewood sign! Being a big fan of Suri Polomare, she plans to rearrange the letters so they instead read "POLOMARE"*! Cut her off, gumshoes, and return the unedited sign to its rightful place!

*The "W" would be flipped and part of the D would be used to turn the "P" into an "R".

5086049 Please, this is the superior version:

5086597 Please, Rockapella made Carmen more known.

5086424 It is a fanart from Anarchemitis (http://anarchemitis.deviantart.com)

5087168 And... what is the full URL?