Let's be honest, the Overworld in Story Mode is really crappy. It is also very old, it has not seen any serious improvements for years. The current Overworld is pretty much identical to the time when Story Mode was added.
The Overworld may be interesting the first time you see it as you can drive around with infinite nitro. But there is really not much to look at, it looks really ugly and has not seen any real change. Compared to the tracks, the Overworld is absolutely sub-par. But frankly, the Overworld was ugly even back in the day when it was introduced.
The Overworld isn't really that interesting and it gets boring fast. Driving to the bubbles is mildly interesting the first three times, but then it just gets boring. The fact that you can click on the challenge icons to teleport doesn't make it really better. Overall, I don't think the Overworld really adds anything to the game and just gets in the way of playing. If the Overworld would at least be artistically pleasing, the minor inconveniences might be forgiven, but that's clearly not the case.
Another problem I see: The Overworld seems to have hold back any serious development and tweaking on Story Mode. Apparently a seemingly simple task such as “add one challenge to Story Mode” appears to be serious business, so it does happen rarely. This would explain why any Story Mode changes are usually just minor tweaks or changes that don't add many new challenges.
I therefore propose to just throw away the entire Overworld and rethink the way how the Story Mode challenges are selected.
I suggest to use a simple menu page for that, in which the progress is displayed and all the challenges can be selected.
One thing that is good about the Overworld is that the progress is visible on the screen at all times. That feature should obviously exist in the new design, too.
With a simple menu, this would open up potential in the future for some deeper improvements to Story Mode and gives more room for experimentation.
Note: I think the mechanics of Story Mode are mostly fine. It's just the way to select challenges that doesn't seem to be implemented well.