So, there's no way for you to change which of your calendars is the default. However, a fairly simple workaround is to import a new calendar that has all of the events from your default calendar. And then remove all the events from the default. So, that way, when you get invites, you can name the default calendar your Invites or Events or something.
To do this:
- Go to your default calendar's settings
- Click the ICAL option for the Private Address and save the .ics file to your computer
- Create a new calendar
- Click the Add dropdown and select Import Calendar
- Select the .ics file you just saved, and import it to the new calendar you just created
Voila! Now you have a new calendar with the original events and you can delete all the things off your default and let it only have the invitation events.