Her role in Nemesis was sound and it worked, but it would break all barriers if she was included in Star Trek XII.
Actually, her role in Nemesis could have been played by anyone. All she does is ask Picard if he wants to go to the Romulan Empire. She mentions nothing about events in Voyager, and neither does Picard. It was just a glorified 20 second cameo.
As for her returning as Janeway in any future installment of Star Trek, I think that would be a horrible idea...especially as the ship's computer. I couldn't stand her grating voice in Voyager (although she would in all honesty work if she imitated the TOS computer's choppy voice...)