Unfortunately, if Giovanni were to return, it would be some unimportant part, if they decided to add him. There is likely to be little plot change. It is a remake, not a whole new game. There is a great possibility that Kanto will be the same as before, unfortunately, so don't get your hopes up about making too much change. With the exception of a couple of characters, but we might be surprised by something new.
A lot of people have high hopes for Kanto and Team Rocket because of the extra amount of memory and space a DS cartridge can hold compared to that of a GBA cartridge. However, I wouldn't expect Team Rocket nor Kanto to be exactly the same as before, but I don't expect it to be drastically different either. Although, there have been some unexpected surprises so far, so I think we may be in for something more compared to the originals.
If Giovanni is brought back like people may want, I wouldn't think of it happening in Johto, but at least in Kanto where he left the team. Like said, the storyline wouldn't change from the originals, so that will stay the same. However, even if Giovanni makes a comeback, just for a battle, then even if it may not be of high importance I will see it something as memorable for the players.