@Linkachu had a lot of the general stats people would want covered!
...Registered users can access a second TCM, and save it to their profile... . The next time you create a Trainer Card, or open up a previous TC, it gives you the option to link a Back to a Front. Then, when the front is viewed, on click or roll-over, it will display the back. ...
I like the idea of having people sign up to use the more...shall we say, "premium" side of the TCM. It has the possibility of drawing in new members, though probably at the cost of a bloated inactive user count. The idea to create a stand-alone second card just for stats does allow for people to easily make a standard back to their cards and apply one card to all their others.
Roll-over on the other hand may only be able to happen through turning the images into a combined GIF. I've seen some sites that allow their users to 'create your own GIF!'. Not usually the most well-known websites and sketchy, but as far as I can tell they do the job-- so it's a possible thing to implement using a web page as the medium.
... I think people would really like on their would be what's in your friend safari 'mons/type. We don't really have a place in our profile that really says what our friend safari is. ...extraneous Pokemon not on your team but in the PC (...in overworld/menu sprite form or something), ...(which game, or if it's for an RP, or a fic, or whatever).
Friend Safari is a great idea to go along with what
@Linkachu said. I also like the idea of the extra pokemon. Through my Pokemon journey, I had at least my Solid Six, but also a few Pokemon that I swapped out. This would sort of give a nice opportunity to show them off! The idea for more or less labeling the card for it's use is a good one, too.
Aside from hopping on everyone's ideas, I was thinking that with the stats (or perhaps the entire back of the card, for that matter) There could be "designated slots" for stats, and you could just choose (with a limit) how or what is displayed, and possibly where. For example, if you want to show off some extra Pokemon, you can select the stats you want first, then your Extras on the bottom. Think it'll look better on the top? Reset the card, then do the Extras first, then set your stats and fill it in.
Regardless, all this sounds like a lot of work, and we certainly may not know the technical limitations of the TCM/coding in place, nor know Teapot's overall goal or want for the TCM-- but I hope these ideas certainly help with the future of the TCM!