Now let me explain real quick what I mean by "commercial". By that, I simply mean my feature would come with ads to produce revenue. The end user still gets free access.
You see, I came up with some ideas for features that would be really nice if they supported, but I would doubt they would support any time soon. The terms of service state that the service must remain free for the user and it must use the unmodified interface provided by Google. So my real question at this point becomes what they intended here as a "modification."
My ideas can all be implemented in such a way where the new feature I want can be implemented on my own web server and will store all the additional information (basically just some meta-data on the regular use of the Google service) required by the feature. However, all actions normally handled by Google's provided interface will still do so. I plan to just simply embed their interface in one way or another so all normal action handled by that interface is automatically directed to it with no intervention. So if it is through a published API I could just use that under the hood and make no attempts to hide it is using the Google service and fully acknowledging its use while extending it with my own API so extended API calls go to my server instead, or if only a web front is available, just embed the web front into my site and simply place my interface for my new feature around the unmodified web front embedded interface.
Usually Google seems to give off a type of atmosphere like they would welcome such development being done for them that take advantage of their services. Much like the way they are doing everything possible to encourage development for the Android phone and selling an app on the Android market that uses a Google service is fine. It only means you pay a $25 developer fee and let Google take 30% cut on the revenue from the market sales. But here I am more worried since I was planning on using their service through a site I controlled instead of Google. Not to mention how some apps have already been developed for Google services that explicitly broke the Terms of Service such as GmailFS.
So I realize this is more of a legal question than a programming question, but it is just about the legality of something I am programming. It is an area I really lack any knowledge in and I figured some of the more experience engineers in the field have come across something like this in the past working for their respective companies if something came up where you needed a feature and weighed in on just implementing it yourself or at least a similar scenario where development resources were not the only discussion but the legality as well.
So, to wrap it up quickly, would it be legal along Google's ToS if I made an app that used their service as required, but then used it with external data from my site to implement custom features?