It's very difficult to see how an anti-PP government can be put together.
1. Ciudadanos and Podemos seem incompatible:
Ciudadanos have said that, because of differences over economic policy, they could not abstain if Podemos is part of the deal. In other words, they'll vote against the installation a government based on a PSOE-Podemos deal.
2. The question of a Catalan referendum is a major stumbling block in the way of a PSOE-Podemos deal:
Both Sánchez and his alleged rival for the leadership of PSOE, Díaz, the President of Andalucia, seem adamantly opposed to a Catalan referendum. Their refusal to accept the Catalan right to self-determination, and by implication the right of any other peoples within Spain (notably the Basques), is a sticking point. Iglesias has indicated that agreeing to a referendum is a condition of doing a deal and even if he were inclined to put that policy on hold (which I doubt), I don't think he would be able to. Podemos votes and MPs come disproportionately from Catalonia, Galicia, Valencia and the Basque Country. In Catalonia, Galicia and Valencia Podemos stood not exactly as Podemos but as part of alliances which include people for whom the right to national self-determination is crucial. In the Basque Country, where Podemos came first in votes, they got lots of those votes from people who might otherwise have voted for the (now peaceful) radical Basque Nats. These people can't easily park the question of a Catalan referendum without accusations of being traitors.