Well after reading up to the recent Kenich manga I gotta say I want to see this crossover. It could go a little something like this.
I think it would be interesting to have Ranma and Keniichi meet up in a respected manner instead of having Ranma replace him. Since due to the recent arc we see how yami is all encompassing. Why not have Kenichi and miu walking home one day when Kenichi overhears Ryoga threatening to kill Ranma. Ranma in a playful mood run away with a smile on "her" face at the time and pretend to be the poor beat up waif. kenichi always a knight in shining armor comes to Ranma-chan defense much to Ranma surprise and Ryogas. However Ryogas does not relent and attack Kenichi to the amazement of Ranma and Ryoga kenichi is extremely capable. He doesn't have anything fancy but try as Ryoga might he couldn't land a soild hit on kenichi to get the "pesk" out of his way.
On Kenichi side Miu is putting on her serious face as Ryoga attack style is brutal and damaging if any of it connects, Kenichi for his part cannot believe yet another strong opponent showed up and wonder where all these young super power martial artist is coming from. He is futher amaze that no matter how much he seem to hit Ryoga the Lost boy brush it off while he is using his Sei ku ability to the fullest to stop Ryoga from connecting and really damaging him.
As we all know Kenichi burn slowly so as the battle rages on Kenichi actually manifest a battle aura and start to figth with more strength and precision much to Ranma and Ryoga amazement. After both teens seem to be inflicting more damage then necessary on each other Ranma decide to stop it before it get out of hand seeing as he "she" was the one to cause it all and with a simple splash of water ended the fight. Kenichi and Miu both get wide eyed at what happened while Ranma picked up Ryoga by the scruff of his neck and chide him publicly.
Ranma gave Kenichi a once over and gave a nod in respect and proceed to walk off with Ryoga in tow.
From there it could go that Kenichi comes home and his master was curious if yami decided to Attack Kenichi, but when they reiterate the fight and how it ending Hayato laughed mirthfully and explain that he too was once under a curse , one that made him a girl. cue everyone picturing the big bulky man in a woman dress and get sick.
Lost of slap stick humor available for this kind of match up but that is how I would start it and then gradually introduce both worlds and their nuisances. We could have Kensei Ma be a sworn enemy of the Chinese Amazons.