Moving away from Jane as the romantic match and then getting Sif in that role should in fact be the result of a well written rationale. I'm not saying to pull a Superman III here and push Jane off somewhere but these two crazy kids have to come to the understanding that this isn't going to work long term for either of them. And THAT is a story.
Not to mention, frankly, they didn't move the relationship or Jane's character forward all that much in TDW. They could have... But gosh darn it, we needed to dedicate all that screen time that would have gone to such unimportant stuff like character building and relationships to physicists prancing about in the nude, TV sitcom stars returning to beloved characters from the first film and all that well thought out villainous back story for the antagonists.

So, we can have as the main subplot, Jane and Thor's relationship as the backdrop to RAGNAROK's main plot if they decide to handle it well and flesh it out.
And... THAT'S what I mean by dropping Jane and moving forward with Sif.