Solo Play is similar to the old version of SuperBetter in many ways - you can access a library of Challenges (formerly called Power-Packs) to play on your own, and you can create custom Power-ups and Bad Guys.
In addition to an updated design, key differences are the ally experience is different, there is not yet a way to create custom epic wins or custom quests inside the app, and you can't upload images.
In the next phase of the product roadmap we plan to add the capability in Solo Play to create a completely customized Challenge including the epic win, bad guys, power-ups and quests. If you were someone that heavily used the customization features of the old app, you could wait for the new functionality, or keep track of your custom quests outside the app.