I'm broadly in the camp that this was always unlikely, imo Franco was too clever to be drawn into a messy conflict, but assuming he could have his hand forced in one way or another (and there are lots of imaginable scenarios) - what effect does Spanish entry actually have?
Arguably it gives the allies another entry point into Europe, with slightly more naval opposition (though not of any particular quality.)
Gibraltar will be besieged, and the Balearics and Canary Islands become something that has to be tackled.
It may also reinflame tensions within Spain, if the remaining Republicans start receiving allied supplies - Spain could be more of a liability than an asset, might require Nazi bailout ala Greece? This is somewhat likely, Franco wouldn't have total control in the cities by 1939.