Well according to the president of the Turkish defense industries, there will be no stop-gap fighter purchase, other than a possible F-35 purchase. Focus is on the TF-X and probably most probably an indigenous F-16 upgrade.
But then again, you never know happens and circumstances might change. IMO low chance of a Russian fighter being bought because of the bad political consequences with the S-400 purchase. They might go for the eurofighter or more F-16's if the F-35 is not available.
TBH, they should've gone for the F-35 as their last fighter and use this decade (2020-2030) to build a formidable missile arsenal with precision strike capability with various ranges and association infrastructure like Iran has done for the last 30 years. Turkey is very similar to Iran in terms of geography suitable for underground missile bases and the like. They might not built an arsenal as large as the Iranians because of the higher prices, but the quality and reliability will most probably be better than the Iranian ones. Higher reliability and quality means a higher Pk.
All of this would mean shelving the TF-X program and go for an F-35+indigenous air-to-air UCAV. 150 or so F-35 with 300-450 wingman UCAV's, with MIUS, Akinci's and the like + a huge ballistic missile arsenal is a very potent, balanced and diverse precision strike capability. And the gained technology of the ballistic missiles would translate to other fields like air-defense and a space program.