I very much like the idea of re-using the fun chocobo activities from the past.

The raising, breeding, and racing activities I think would all be enjoyable.

I'd like to add another idea, that of mounted chocobo combat. Many of the cut scenes show mounted chocobo knights, especially from the Ishgard area. Letting players become chobobo knights themselves would add an entirely new dimension to the game.

My thought is that rather than making chocobos grant immunity to attacks, let combat be possible while riding chocobos. Chocobos could have their own HP, defense, mag defense and evasion, and their bred stats as well as armor would determine these stats. Many of the later game monsters are incredibly fast already ... antelope stags and raptors come to mind. A grass Raptor is as fast as a gatherer with the gatherer speed buff, so making chocobos as fast as that wouldn't be terribly unbalanced (in addition to the spectre of AoE attacks).

To help balance chocobo combat, perhaps TP generation could be reduced while mounted, and MP regeneration could be turned off while mounted, perhaps even stamina generation would be reduced. Specifics, of course, would depend upon the details of how the battle system is being revamped.

This could all lead up to a new job (not class, but job, assuming some sort of two-tier job / class system), the job of Chocobo Knight, which would add special abilities / bonuses when fighting from the back of a chocobo.

Special bonuses could include negating combat penalties from basic chocobo combat, granting additional HP or defense to your chocobo, or adding new attack 'weaponskills' that consist of chocobo attacks such as "Charge" (like Raging strike, could add bonus damage to a weaponskill), "Peck" or "Talon Strike", as well as a "Chocobo Sprint" ability. I'd like to see some abilities for gatherer classes to synergize with chocobo riding, such as "Dig" for mining or "Graze" for botany.