It would have been logical for Turkey Air Force to opt for the Eurofighter or the Rafale for that matter. I don't see the Viggen an option though. The Su-variants are better alternative but it carries some risk. Turkey being a NATO member leaves it with no choice but to stick to NATO standard. Not to forget the current procurement of the S-400 drew backlash from the US and might hurt Turkey future arm deals.
Rafale was not possible cause French Armenian and PKK policy and severe differences between the countries.
Eurofighter was an option, comparable to A-400 M cooperation. But I can't remember that Turkey was ever invited to participate.
Viggen could have been a good choice regarding cost - efficiency and NATO standarts.
Who knows whats going on behind closed doors ?
For sure Turkey needs a stop gap solution until TF will be operable in significant numbers. ( 2030 ?)
Back to topic, it seems that HMS Ocean will be sold to Brazil. There are 3 years left for LHD/LPD Anadolu air platforms equipment.
It is even not sure that next year first two F-35 A will be delivered to Turkey. F-35 B is not on the official agenda.
Let's assume that Turkey makes a real U-turn and cooperates with Russia on a new YAK-14(1) version ? (Wishful thinking)
Russia is pushing production of new Aircraft Carriers and derivatives.