Two of Marvel’s most enduringly popular characters who’ve managed to transcend their roots as capitalizations on exploitation cinema, Luke Cage (formerly Power Man) and Danny “Iron Fist” Rand have operated separately but are likely most known for their co-starring role in the Heroes For Hire series.
Cage is possessed of virtually-indestructible skin and considerable superhuman strength as well as being fairly handy in a rumble, which is helpful given his tendency to find (or create) trouble while preserving the safety of New York’s citizens. Iron Fist’s abilities are a little more rooted in the supernatural – he wields a mystical essence that can be harnessed to sharpen his focus, enhancing his already significant prowess as a martial artists, with the drawback that it has a finite supply and requires time to recharge.
Your story potential here is simple enough – have these cats prowl the streets, kicking ass and taking names and helping the helpless and so on. If the four series were to serve separate functions, and they damn well should, this would be where the bulk of the action would reside. Naturally there’s crossover opportunity aplenty between the two, which extends to the pair’s friendship with Daredevil and Cage’s eventual marriage to Jessica Jones. Luke Cage and Iron Fist would arguably work best as a single series, but if Marvel’s determined to handle them separately then the contrast between Cage’s street-level mercenary antics and Rand’s martial arts adventures should make the inevitable team-up episodes that much more dynamic.