This is a summary of some of those mockups and suggestions, with a few images and links to their original G+ posts to see the whole ideas and reactions to them.
Some of them were simple and intended to avoid common problems. I suggested moving the Send button to the right side of the People box in order not to hide it under the dropdown list, I also suggested simple ways to improve posts' looks by not showing bare-text links; way to avoid full names in mentions (when possible), posts in languages the user doesn't understand, and content duplication. I also proposed sharing any webpage through the +1 button long before that was implemented.
Other mockups dealt with improving the overall Google+ experience: I proposed making hashtags more beautiful and usable, and using them to filter incoming posts according to topic and avoid unwanted content. I proposed enhancing the social and emotional involvement in photo albums by making them collaborative, and improving Google+ as an interpersonal communication tool by making it easier to contact a specific user and find previous conversations with them. I proposed encouraging original content by using a smarter sharing model, and I suggested using the authorship markup to give brands and content creators a reason to have a G+ profile and spread the use of the Add to circles button across the web.
Improving integration between Google products has always been one of my main goals. When Reader was overhauled I made an alternative redesign, more in line with the new corporate look. In the same post I also created the concept of "shelving" to address one of the main complaints of Reader users, the fact that they lost content downstream because they could not save or categorise posts in Google+.
I also sent several mockups proposing different levels of integration with other Google products like Docs and Calendar.
As for the newly launched brand pages, some time ago I made a few mockups showing how brand pages, Places and Products can work with Google+ and each other so that both users and brands get a better, more integrated experience regardless of which Google product they are coming from.
Google+ is not the only Google product I have sent ideas for: as an Android user I often come up with ways to make the user experience better, such as using available data from Google+ and Google Maps to tell the user who's calling even if it's not a contact.
You can find additional examples of my work, users' reactions and reshares by doing a search for #googleplusmockups on Google+, or you can check out my redesign of Google Tasks.