Even if Arda proves himself we need midfied organizer badly. Rakitic loses too many balls to fulfill that role.
We just won a treble and have like 90% of wins percentage since January 2015'.
I personally wouldn't want Pogba here, because Rakitic would need to sit on the bench.
(Pogba+Rakitic midfield doesn't make sense. Rakitic is not a playmaker, and Pogba is a clown of a playmaker.)
About Gundogan, Rakitic+Gundogan is probably too defensive again, we need an Amc-Mc like Iniesta.
And if you want to bench Rakitic for Gundogan, again, you will win some things, and you will surely lose some skills.
Again, you Rakitic was a very important player in our revival and in our treble.
He has a nice connection on the right with Messi and Alves.
Gundogan for the future and for a rotation currently?
Well, that is a different story...
We don't need a starter right now.
(Oh, I realized now that I totally forgot about Arda in my post, huh...
We are planning new "starters" and we have no idea yet how will Arda play.
Maybe Arda-Iniesta-Rakitic(+Rafinha) will be more than enough for the next 2 seasons.
What if Arda turns out to be awesome and he kicks Iniesta from the team?
Where would we fit Gundogan then with Turan+Rakitic on their prime, while we would have Iniesta and decent Rafinha on the bench?
If Turan turns out to be awesome, we will already have too crowded midfield even only with Turan-Rakitic-Iniesta trio.
= for example, MSN trio needs to play all matches, lol.
Now, if we have Turan-Iniesta-Rakitic, they will already play only 66% of matches.
Add Rafinha into that, and they will play even less than 66%.
There is no room for another midfielder currently if Turan will turn out to be good, and if Iniesta will continue to play good.)