How about Memrise introducing a learning index ?
That would reflect how effective we are at learning!
I mean, I noticed reading different topics that a fair amount of users (and “cheaters”, despite cheating at learning is a strange concept to me :p) misuse the app and are around more for the point count rather than actually learning.
Technically, the less you have points per word, the more effective you are at remembering them because it means reviewing them less often and them going to your difficult words list less often…
So in addition to the current badges, having such an index could be as well stimulating, to keep up with effectively learning rather than looking for just reaching high ranks (which is fun as well, but I find it more challenging and stimulating to reach these ranks knowing that I can really be able to speak rather than knowing that I’ve reached them because of reviewing 147 times the word “no” or “whatever” in 7 zillion languages over the course of 2 years :p).
Well just an idea that crossed my mind, not directly useful I know, as all statistics available in the pro version, however indirectly useful when you like challenges and to be around to learn effectively and not just to get points 
I would simply calculate it this way :
Index = Number of points / words learnt / number of days days since 1st word learnt on Memrise
Well just a random idea 