I would go for Valles in January or Merit.
Someone who isn't essentially replacing Ter Stegen but can directly compete with him, that coaches won't be afraid of starting them if they feel MATS is in bad form.
Both has expiring contracts, so won't cost an arm and leg, will address the biggest weakness we have (backup GK) While potentially fixing the starting one.
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PS:
1- I won't rely on La Masia, Barca system doesn't favour GK development at all. The likes of Astralaga or Kochen aren't anywhere ready and we will keep them stagnating in 3rd division or on our bench. Kochen got serious injury btw.
2-We were linked with Backhaus, but after returning to Bremen, he failed to be starting GK so far. Maybe it was an agent PR, or we are actually trying to get a new young GK.
For a young GK there is Restes in France but he is anti Barca tbh.
3-If we are going to splas big, Bart Verbruggen & Trubin are worth consideration.