Fenix, about Gazille's inclusion, I'm thinking he and Levy are more likely to pass than Gray and Loki because his importance as a dragonslayer may come into play with the new mystery guy if he turns out to be one. Naturally Levy's the one to really move on, but Gazille comes with. He's the only reason why she still has a fighting chance in this trial.
Chaos the next exam could really be anything. This 1st exam is meant to test the combatants strength and luck, and Cana's team having the luck to run into the only other team who would willingly surrender to them, and Natsu using all his inner strength to not outright defeat Gildartz, but to accept his limitations and inability to defeat him are good examples of that.
I guess you can also call Natsu lucky because he fought a guy who has a sense of favoritism towards him, but it was his maturity to call off the fight once he realized he didn't stand a chance which showed true strength of character. For a guy like Natsu who over 200 chapters hasn't given an inch to any bad guy whatsoever that's major character development.
I'm guessing the next trial will also test some kind of attribute. Possibly their intelligence, or something random. It could really be anything. I doubt it'll be more fights, and Fairy Tail's 1st master Mavis Vermillion is supposed to be resting on Tenrou Island. What that means is anybody's guess but I doubt they'd make mention to it for no reason. Mystery Guy still has to make his grand entrance as well.