Sounds like you've hit the perfect spot! Personally, I’m more into the best friend's mom scenario – a little forbidden, but not too over the top. How about adding an unexpected twist or some humor to the roleplay? Could make things even more fun!
wrt (step-)mom and (step-)son, when y'all have done that role play, who has the upper hand / control - the mom or the son?
tgere are lots of various scenarios, including ...
* one want to coerce/blackmail the other (for something they found/saw/heard/found out/etc.)
* one wants a favor from the other so has to "submit" to the other
* one is just horny and "attacks" the other who at first says "no way! it's weird!" but then gets into it.