A Turkish divisional effort to hold the Al Bab plains and to create a 'no fly zone' for their clients to stage in, possibly. I'd be very amused with that though since it would make them fair game for the Russians and Iranians. Of course they could widen the conflict and that would be OK too. Unless they can somehow convince NATO, or more than say Poland and Estonia to go all in, it might just get a tad unpleasant.
Of course the alternative is to accept the defeat of their proxies by the unholy alliance of heretics and unbelievers. In this case it would be best if all the Syrians and foreign fighters were to die gloriously in Syria. It would be quite a problem if tens of thousands of militiamen were to come back into Turkey, especially if they did so with some of their equipment. Disgruntled fighters with small arms will make southern Turkey a pretty unpleasant place to be. Yes, they could be mobilized against the PKK but I'm not sure if that really is a winning hand. Otherwise you have many battle seasoned military age males with no prospects, no money and much frustration.
The Syrians might be bled white, there are many Iraqi Shiites ready to take the fight to the enemy. Khataib Hezbollah may not be as well trained and seasoned as Lebanese Hezbollah but they are very numerous and they are well equipped. Stiffened with Saberin and supported by Russian voenspets, they have been very effective in Aleppo.
The notion of a Saudi intervention through Iraq or Jordan is really even more hilarious than their showing in Sadaa, on their own border. I think it would be very amusing, since it would collapse logistically long before they got near any Syrians. Still, the prospect of Hezbollah M1A1s and Saudi M1A2s getting into a brawl in Anbar is very attractive.