There are plenty of third party 8 and 16 bit hardware on the market. Retro-bit looks to stand out by releasing the Super Retro Trio and Super Retro Adapter for Game Boy Advance games. Lots of other third party systems will use their own controllers and it won't quite be the same. SRT will have plug-ins for Genesis, SNES, and NES specific controllers. There are six to be exact, that are compatible with first and third party controllers. The system will ship with its own pair of controllers, but it's important to be able to use your preferred setup.
The system itself is 3-in-1 as you can leave your games in the system and swap between them. The system does support s-video, so this allows a bit more clarity on the games. The Super Retro Adapter will be sold separately as this will support GBA games. The option is available to actually plug this into any Super Nintendo or third party SNES compatible console. The SRA has a link cable port for connection of up to 3 Gameboy systems for 4-way games, such as Mario Kart.
As of this time, nothing is confirmed about using interchangeable controllers for other systems. An example would be playing a Street Fighter title for Genesis with a SNES controller. This would be nice to have. As for the SRA, it's compatible with GBA games, but not confirmed for original Gameboy or Gameboy Color titles, as the original GBA was. The SRT will retail for $69.99 and the SRA is sold separately for $39.99. Look for a hands-on after E3.