Unless it's intended to be a multi-game protagonist from the start (a la Shepard), I'd rather just start a new game with a new character.
Who says it wasn't planned? I always felt like there was a reason the Inquisitor and Inquisition were so safe in the base game, and now in Trespasser as well. Bioware has trolled, mislead, and changed their minds about things before. It's not impossible that they wanted to make an Inquisitor sequel for a while. I hope, anyway. Honestly, I think they were going to have Hawke be the Shepard of DA and have them become the Inquisitor (especially since a lot of the cut Exalted March expansion ended up in DA:I, like the Temple of Mythal). DA2 just didn't do as well as they hoped and they panicked. My point is, I think they actually wanted to have a returning protagonist before. Also, there is a new lead writer now, so anything is possible (as he keeps saying, himself).
The Inquisitor is the worst choice to go up against Solas now, because as others have stated Solas knows him/her. That's a priceless advantage, and the IQ still doesn't know as much about him.
The Inquisitor's story is done, just as the HoF's and Hawke's. Someone new will take the stage, and Bioware will tie it in.
1. I don't really understand why it's so important that Solas knows the Inquisitor. Quizzy still goes up against him no matter what, and wouldn't guiding the offensive against Solas be much more of a risk if he knows you than simply going into the field? If anything, it's backwards.
2. Why do you keep saying the Inquisitor's story is done? I don't know how to say this more simply, but did you watch the epilogue? It's clearly not done. Plus, people said the Inquisitor's story was done in the base game, then Trespasser happened. Anything could happen at this point.