Well nothing is definite yet, but I think you are too quickly discounting the concept of a pair of forums nestled comfortably together under the POTN mast head, each with there specific system in mind.
For the Canon shooting POTN shooters that switched to Noink, I can see the appeal for keeping it all together, but lets not forget that reason POTN is so great. It was the members that helped guide the atmosphere, and grew it to it's current state.
If we add Nikon forums, than the Canon Digital Photography forums will cease to be what it is, and we will be emulating every other tom dick and harry mega forum out there, complete with all the cross posting and in fighting and flame wars that are inherent with locking up two differing species in the same cage.
A separate but equal forum will allow many advantages:
We will remain unique.
We will allow the membership once again to take a significant lead in the growth and direction the forum goes in from day one, as opposed to new Noink shooters inheriting the existing POTN. (The direction may in fact differ from POTN, that will grow organically. )
Currently the vast majority of the growing number of Noink shooters here were either Canon shooters first, or had switched to Canon for Digital in the past,.
With the introduction of Nikon forums, either separate or part of the current forums, that would change. We would quickly attract the long time Noink shooters as well.
In no time, I feel that the number of Noink shooters NOT interesting in Canon gear would out grow our current ones that do.
Marketing options are also something the owner has n mind, separating possibly conflicting marketing approaches is simply good business sense.
Lastly, if at some point the idea of merging them seemed to be the more logical option, this would be FAR easier to do than separating the two entities,. in fact separation would be essentially impossible.
Launching it as a separate yet equal forum side by side with the current Canon forum poses no risk to the current Canon forum, none at all, and leaves the fates separated.
I understand for some it would be "easier" if we simply added other equipment forums,.
But frankly I do not think the "easier" option is the correct one in this case, life is often that way. the harder option is often the one that offers the best end results. So we need to be ready to apply extra effort to get the best new POTN Nikon forum.
Choosing the easy way of adding new forums is not the safest option, it's not the most elegant, and it's certainly not the unique concept that the other option is.
As for it becoming another "nikon cafe" I have no personal experiences there, but I do know that the POTN members shooting Nikon now, if they are willing to jump in with there boots on and help set a standard in a new fresh (and remember, VERY SMALL and there for VERY manageable forum) if they are willing to put there money where there mouth is and lend a helping hand in guiding new members and moderating the new forum with care and knowledge of the Nikon system, then you are the ones that can make the new POTN Nikon forum what it should be.
IMHO it will be a great opportunity to once again create a forum from the ground up that will put the others to shame.
So don't think about what will be easier, instead think about how YOU can be a guiding member of a new forum with a stake holding share in it from the ground floor.