Well I do agree on the fact that it could be Stella's friend, but I dunno. Outside of Robin and what.. one other? We haven't seen many of Ted's ex's, so her showing up, especially near the end of the season could prove to be important between them.
Or you could look at it another way; Sarah Chalke won't have much time to come to the show if Scrubs gets picked back up for another season (which it shouldn't), so it'd be hard to squeeze her into another season. I feel we'll find out about the mother with the wedding and what not, then a flashforward to the current Ted/Barney/Robin/Marshall/Lily, etc. That's my thoughts anyway.
There's been evidence about something in Ted's room belonging to Stella so.. that's sort of odd. I honestly don't know where they are going with this; one of Stella's friends that Ted gets introduced to somehow, or her and Stella get back together after something not going right with Stella's ex.