It looks like the Sega Game Gear will be the next system supported by the 3DS Virtual Console. Japanese blog Zettai Simple Shugi got its hands on the latest issue of Weekly Famitsu, out every Thursday and often the first place with gaming announcements in Japan, and the magazine reportedly says that three titles will be on sale from March 14th. The games on offer include Sonic and Tails 2, Shinobi, and Dragon Crystal: Shirani's Maze, all going for 300 yen (about $3.75) each. Despite Nintendo recently starting to sell downloadable NES games for the 3DS Virtual Console, the Game Gear's addition would suggest that the priority for the service is on portable consoles — those wanting games from more popular home systems such as the SNES or Genesis will likely have to stick with the Wii's equivalent service for now.
We're currently tuned into a Nintendo Direct broadcast here in Japan — we'll be sure to update this post with any official or additional information that may come out of the briefing.
Update: Nintendo has indeed confirmed all of the above.