Well at first this would seem like a perfectly reasonable suggestion to make as there is a cabinet for downloading retro city rampage in the arcade section of Ms. Splosion Man despite having no known attachment Ms. Splosion Man or Twisted Pixel in any form. Retro City Rampage has already confirmed that guest characters will be appearing in the game, I can't remember off the top of my head but I know they announced a few, the only one I remember for sure is Mr Destructoid.
 
Well I took the liberty of unlocking all of the concept art (barring the completed game concept art, due to the fact I haven't yet completed the game) and if you have also unlocked some of the art you will notice that when you click "A" on any art piece you will get a brief description of the art. Well if you unlock one of the last concepts of three alternate facial expressions of Ms. Splosion Man you will notice that the description informs you that the concept art was created because Ms. Splosion Man will be appearing in a title from another developer and alternate expressions were required.
 
Just my own personal observation but I am going to go ahead and CONFIRM that Ms. Splosion Man will be making an appearance in Retro City Rampage.

@craigbo180 said:

Just my own personal observation but I am going to go ahead and CONFIRM that Ms. Splosion Man will be making an appearance in Retro City Rampage.

Bold words.