Nintendo UK has announced the Yoshi Chase event, which is a chase to try and get all the different colored Yoshis in Mario Tennis Open. This activity will have you do different things such as going to a certain store to find QR codes to scan so you can earn a new colored Yoshi. Check out the official Nintendo UK page for details of this event. This has only been announced in the UK for now, but I would think other regions will be doing something of the sort.

Source: nintendo3dsblog.com