It seems to me that the great hope of the Spanish center is now the mutually assured destruction pact that a PP-PSOE coalition would represent. Actually, this is almost certainly the great hope of the PP which wouldn't stand to lose quite as much as the PSOE (whose slogan in the last election was "Let's kick out Rajoy!"). But therein lies a clue to the potential pact: like the CUP in September's Catalan elections, the PSOE hasn't said no to any PP candidate for president. It has said no to Rajoy, which implicitly leaves the door open for an alternative candidate. Soraya Sáenz de Santamaría would appear to be the obvious choice.
So that's one option. The other is a center-left alliance of PSOE and Podemos, which would also need the support of some regional parties to rule. Which would mean the PSOE offering a Catalan referendum, which Sánchez probably couldn't offer even if he wanted to.
I suppose the difference between the Spain and Catalonia situations, vis-à-vis the question of negotiations to form a government, is that the Catalans have the advantage of a seriously big question, a national project, which dominates and blurs party politics. This is, at least in part, intentional. But it's also helpful because in the end, there are enough people who actually believe in that project that it can be used to forge tough political agreements, like the CUP forcing Mas to step aside and then backing one of his proteges for president. Spain has nothing remotely similar on the table. You hear terms like 'constitutional reform' and 'new transition' bandied about but unlike Catalonia, where 48% of voters voted for unambiguously pro-independence parties, the 4 main parties at Spain level don't have a coherent vision of the nation to offer voters. Even the upstarts – Podemos and Cs – have been unable to explain to voters what Spain looks like in their vision of the future. This is either because they don't really know or don't really care… I suspect it's a mixture of the two, personally.
In the end, say what you will about the independence process and its putative ephemerality, at least it's a project. Spain has yet to come up with something similar and the best options for change – Podemos and Cs – don't have the support. So it's Soraya for president and continuity, or new elections with nothing any clearer.