at the end of the game after the credits u hear travis telling silvia "santa destroy needs us"  
 
heres what i think for nmh3 (hopefully i it doesnt sound like a garbled mess) 
 
travis tells silvia that santa destroy needs us. because now santa destroy is a mess 
 
there are tons more assassins aiming for travis title of number 1  
and each day they are just either killing civilians of santa destroy or fucking up the place each day  
so now he goes and finds silvia so that she could organize the assassins into fights (didnt she basically do that ) 
 
something of that sorts.  
the first no more heroes was primarily about travis, the second no more heroes was more about santa destroy and assassins really, so the third one could be about "Paradise" (that has been the recurring theme in the second game) 
 
something of that sorts the third game could be, its worth noting that i just finished the no more heroes 2, so im just riding a brainstorm im having :P

Suda 51 has stated that this is the last game Travis will star in, so don't get your hopes up for seeing him star in a later game.
 
A cameo is possible though, I imagine.

I suppose it could be about Shinobu. After all hte game never reveals what the deal between her and Silvia was. Silvia her state during the cutscenes (the phone boot brothel scenes) might be caused by Shinobu after the final fight against Bat Jr.
 
It might also be the story of what i presume was Silvia her daughter in the true ending of the first No More Heroes.