Are El Niños stronger when there is a longer gap from the last El Niño?
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.