I found an article from back in the TASM1 production days suggesting that Lily did an early 'test read' for the Gwen role, which would make for some neat symmetry if she were to audition to replace Shailene - should they need to hold auditions - and get the part.
Another Gwen 'test actress', Emma Roberts, would also be a viable contender; she's prettier than Shailene and Lily, but not to the point that you couldn't buy her as an MJ who has elements of Ultimate Gwen to her characterization (which is the direction they took based on what we know about how Webb and Co. approached the character).