Decent young mid tier blokes that may seek further opportunity elsewhere that I can think of are:
Jesse Lonergan (GC)
Alex Sexton (GC)
Adam Kennedy (GWS)
Nick Haynes (GWS)
Jacob Townsend (GWS)
(As a sample from just GC & GWS)
All 5 of these blokes would possibly already be in the best 22 of every other club, but aren't at their present club due to the depth of talent around the same age. I'd have any of them TBH
Pretty sure it hasn't been an issue for Haynes, as he's best 22 for them - he's just been injured a lot. He played the first 8 games last year I think - then went out injured for the year I believe.