While I disagree that our biggest weakness is the midfield position (we're weakest at RB), I completely agree about going after Gundogan this summer. As you say, he's unlikely to become cup-tied next season, and if he is planning to leave this summer anyway, Dortmund could enjoy him for half a season longer before he moves to Barcelona if they reach this agreement with us. I think this would place us in a better negotiating position than other clubs, especially if they were to get relegated (something I really, really hope they don't get), it would be good for them to have him for the first half of the season while still having concluded a transfer.
I haven't followed Dortmund too closely this season, does anyone know what the status of Gundogan is right now, has he said anything about wanting to stay?