I think using characters from other comics is generally a bad idea.
It always makes the work they appear in seem like the lesser work, and where is the need?
Maybe in general, yes, but there are a lot of comics that do it regularly. The trick is that if the comic is known for it, it's audience already generally accepts it. PSI has been doing it since close to the beginning, and was even apart of the original plan I had. These days I'm trying to integrate it more, WITHOUT world jumping in most cases. Or if world jumping is apart of it, avoid the 4th wall crap.
The point was to have fun (as PSI is a hobby comic), and connect with other cartoonists, etc. Also I LOVE crossovers and such, and world jumping. The webcomic will continue to have such cameos and crossover. When/If I print, things will be mostly different.
The main answer to these concerns are this: PSI was an experimental, hobby comic from the beginning, original meant to do cameos, parodies, and crossovers. Things have changed and improved all over, but I love the cameos and crossovers, and will continue to do them. However, I will continue to use PSI as a learning comic, and improve my storytelling skills to intertwine everything with the story that eventually developed from PSI.
PSI is my continuous project. It helps keep me sane as a creative outlet.
Why not just invent an original character instead to fill the same role?
to be honest... when I do the rewrite, some existing characters' roles will be extended to cover various cameoed characters' roles in the story. Some existing characters' roles will also be extended to help mold the story better, and streamlining certain things to avoid runarounds.
In some cases, such as the Reaper, I already have "invented" some characters to fill the same role. This will be explained in-comic soon.
In the rewrite, the crossover wars will likely be entirely written out. However, it's possible I'll still write SOME sort of dimensional mishap to bring in some characters... but it'd basically be all new.
The long term implications of two works existing in the same universe just provide further headaches for continuity.
I just cant see why you would want to go down that road.
The idea is that Webcomic PSI will continue as it has, while Print PSI will streamline PSI's 300+ pages into something smaller, simpler, and more to itself. Webcomic PSI should mostly continue as it has with cameos and crossovers, but the main story and plots would relatively be the same as the Print version when the Print version catches up. Basically they'll more or less share continuity, with little differences.
The other thing is that PSI is impossible to print as it is. On TOP of not having enough original material that is actually printable, without re-inking, scanning, and coloring the majority of strips... the Cameos and such are a legal nightmare... I'd also like to make PSI all "mine" too...
I could just up and start completely over. Stop the webcomic version, and just focus on a rewrite... but I can't bring myself to do that. All that time and effort and progression to just up and quit? Without finishing the storylines and plot points I've been waiting years to get to?
I also had another idea to make some stories that would work in EITHER the Webcomic or Print versions... Like I said, aside from the first 200 or so strips, the basic plot and story of PSI is basically going to be the same for both versions.. as long as I don;t reference past events that only exist in on or the other's version.
It all makes sense to me, anyway

I mean, if people are STILL reading my comic after what I have in the archives, that's good enough for me, as long as I keep improving my storytelling.