I would vote for Chomper, he can communicate with Sharpteeth, which since the Gang of Five always encounters Sharpteeth, Chomper's linguistic skills could save the Gang from having to run and hide so much. For example, in LBXIV, had Chomper been with the Gang, he could have either distracted the Feathered Sharpteeth or convinced them to move along (granted they comply, the Allosaurus in LBTV did not comply).
In my opinion, the end of LBTXIV basically indicates its going to be The Gang of Seven, with Chomper and Ruby joining the pack. My concern about this is that with so many characters the amount of time spend on each individual character will be less, and it is the original Gang of Five that has been through every adventure together since the Original Film; it is them that we have connection and catharsis with. Chomper in contrast to Ruby, has more history with the Gang, he being in LBII and V, and the series makes him more of regular than Ruby.