Seems he‘d rather stay in Hamburg with St Pauli if possible as he‘s had a good spell there and feels trusted and appreciated by the manager fans alike.
This (rapidly translated) from Transfermarkt.de (27.05)
For Stojanovic one thing is clear – his future is unlikely to be with Middlesbrough, at least not as things stand. "It was a difficult time for me at Middlesbrough“ he says "the current manager didn‘t seem to rate me which is why I looked for a loan move to St. Pauli“
As Middlesbrough recently extended manager Niel Warnock‘s contract a return wouldn‘t seem a viable option for Stojanovic. In addition, it seems that there has been little interest in North Yorkshirte to pursue the possibility of Stojanovic‘s return, even though their current keeper Marcus Betenelli is just on loan and has not been offered a long term deal. The Austrian added that he had been in touch with the goalkeeping coach at Boro but the discussion was more generally about how things were rather than any specific contract issues.
For what it‘s worth from the little we saw, I actually rated him and – fun fact – the website reckons his market value is Euros 600K