Let me establish one little thing first. The Doctor/Rose ship is my absolute favorite part of the show. I'm a HUGE shipper, and am actively involved in communities centered around their romance. I both read and write fanfiction for them, as well as listen to fanmixes and watch music videos designed specifically for this obsession. I will argue until I have turned blue and then red and then purple in the face, that cannon 100% supports the fact that they didn't just love each other, but were in love with each other. I reserve the right to believe that they will always have those feelings, regardless of whatever pulls them apart at the end of this coming series.
That being said, please let the Doctor move on. If it is true that the woman in the next episode will be a future love of the Doctor, then let him love her. Him eventually moving on and healing enough to be intimate with someone else in no way trivializes what he had with Rose. That's like saying a widow's first marriage meant nothing when she marries someone else. The Doctor and Rose were ripped apart by forces they had no control over. And whatever happens at the end of series four to split them up again, it never would have happened if it weren't for Doomsday in the first place.
And let's not forget the operative word in the above statement. If she's real, she will be a future love of the Doctor. He doesn't love her now. He may not love her for another few hundred years. He may have an entirely new body and Rose could be long dead by then. It's also quite possible that history is going to change and this future flame will die at the end of the episode. We don't know yet.
What we do know is that Rose isn't the companion next season, so she doesn't stay with the Doctor. One way or another, there is no happy ending for them. And as beautiful as it may be, the idea that he goes to live with Rose until she dies just seems like a naive bit of wishful thinking, more idealy suited to the world of fanfiction than than an actual script.
I love this show. Having Donna as a companion has shown me that I can enjoy watching it even without a romantic element. I'm not going to stop watching just because Rose is gone. I watched all of series 3 and series 4 is even better, even without the anticipation of Rose's return. There are so many other amazing things about this program to enjoy.
But even though I'm going to continue watching, I will be ridiculously sad when the Doctor/Rose story arch is over. But I think it has more to do with the fact that what I've spent my time doing these past few years will not be as important as it once was. The stories that I worked so hard on, am so proud of, will not be read. Many of the friends I've made will fade away.
But I refuse fade away myself. As I mentioned before, what is fanfiction for? It is for the fulfillment of a fantasy. We can finish the unfinished, change the unchangeable, make happy what is devastatingly sad. My AU works of (technically illegal) fiction will still be written, even if the audience diminishes until only comprised of my nearest and dearest friends. After all, people still write stories bout the fourth Doctor and Romana. Why can't I have my Doctor/Rose that long. At least in my mind, she really will stay with him forever.