Are El Niños stronger when there is a longer gap from the last El Niño? E.g. see http://www.wired.com/2014/04/el-nino-effects/.

I'm thinking.. In case a major El Niño event didn't happen this year, then would it only make it more likely that a stronger El Niño would happen in a subsequent year?
One issue here is whether El Niño (ENSO) is chaotic in nature. There is a fair amount of literature on the chaotic and deterministic nature of ENSO (e.g., https://www.sciencemag.org/content/264/5155/72.abstract, http://onlinelibrary.wiley.com/doi/10.1029/97GL53639/full). Under the chaotic assumption, the posted question might not have a clear answer.

Then there is the fact that ENSO affects the atmospheric-oceanic energy budget. If too much energy is available, then El Niño might happen, but whether it is stronger is unclear.