When I got the 37 sensor kit a few years ago I figured out what was wrong with how normal arduinos are used to built circuits. Using these little sensors and devices mounted on tiny break out boards with connectors already in place, it was easy to connect them back to a breadboard for power and ground and wire up the data line to an Arduino data pin.
The normal pinouts on an Arduino are difficult. You can't even mount a normal Arduino board on a breadboard (and visa versa), because one set of the pins are wrong. Every model is lacking all the 5v and ground connections a normal project needs. The normal board is too big and the small boards have strange pinouts as well.
What we need is a board that will have rows of dozens of connections that accept a clip in connector. And then have sensors be on tiny boards that accept the other end of that connector. I can see a 5 pin connector being used. ground, +5v, +3v, digital, and analog on the first 7 connections, or however many the built in chip supports . After that just have 4 pin connectors without the analog. If an Arduino Mega was used for this kit you could have 16 analogs inputs. A Raspberry pi with an 8 or 16 port analog to digital converter could work too.
Have various length cables with the clip in connectors on both ends for building various projects. This is similar to the tinker kit connectors, but with every option in every connector so you literally cannot hook it up wrong.
Come with a few simple devices already wired to the boards, just like the 37 sensor kit did, but make these connectors ones that clip in and are all wired identically, ready to go. Have extra blank protoboards in the kit to let people build their own devices and connect the same way that the standard pins connected. Release the eagle files to let people build their own protoboards and designs as well.
I would have the device and boards sit in a plastic carrier that is compatible with Lego, letting you build your projects on the Lego base plates. As you get more advanced with your projects you could still wire up all the individual digital and analog pins up to a breadboard (also in Lego compatible bases.) Additional kits could be sold beyond the base kit to explore different technologies. You could even have spi connectors with boards that let you daisy chain the boards together, but that used the same connectors and pins, but were wired a little differently.
As for the software, I am picturing an extra program that you just say what you have connected up, and which connector you have it attached to, and then you push a button, it creates an Arduino project with the right libraries installed and names attached to the correct pins, and everything already initialized for you, and starts Arduino software loading that package to let you go from there.
Think of this project as "Arduino For Everyone."