Spain 1936 + Durruti & - Franco?
Well why don't we have a switch of fates here a bit of trade off by the force of death. One life for another. If Durruti survives then why not have Franco perish in his place. Opening up the possibilities of a more profound anarchist movement.
Again, sorry to be boring but how precisely is Franco going to die? He was not one to put himself in the firing line, unlike anachist military leaders such as Durruti, there is no record of the republic ever trying to assasinate Nationalist leaders nor of that happening within the Nationalist side if one excepts the unfortunate accidents that occured to Generals Mola and Sanjurjo, who both died in airplane accidents, (Franco has certainly been suspected of complicity in Mola's death). I would guess that the only way that Franco would have died would have been if the flight of the Dragon Rapide which bought him from the Canaries to Morocco in July 1936 had crashed.
A Franco death at that point would have been very significant, more significant I would suggest than the survival of Durruti I would suggest as it is hard to see other senior figures on the Nationalist side at the beginning of the rebellion such as Quiepo de Llano, Mola or Cabanellas, acting with quite the single minded determination that Franco did.
So say Franco died in July 1936, what happens? Probably another figure from the foreign legion would lead the Army of Africa over the straits, Italy and Germany would still support. Ironically given that Franco is normally credited with the decision to pause the advance on Madrid to relieve the besieged Alcazar fortress in Toledo which gave the republicans, including the international brigades and the Durruti column time to reinforce and defend Madrid, then Franco's death could have led to a quicker advance and possibly the Nationalists taking Madrid sooner.
Actually given your fervent anarchist sympathies, I would contend that an earlier Nationalist victory would have left the anarchist movement in a better position as its defeat might have been less profound than it was by 1939, but that is probably unlikely, the mood of 1936 was not one that offered a compormise settlement on either side.
The other possible premise of an early death of Franco is that the war takes longer and possibly there is no outright Nationalist victory, possibly even that the aim of Juan Negrin that the republic survive till the outbreak of a general war in Europe when they would receive more positive support from the Allies. Again, given the lack of international support that anarchism could command then I can't see it being a very positive outcome for the anarchists.
What the Spanish anarchists achieved in 1936, particularly in rural Aragon was extraordinary but it was the function of the chaos and collapse of order of the time and the rooted nature of Spanish Anarchism, the longer the war went on the more it became apparent this was a high water mark, with or without Franco I can't see Durruti changing that.