Yep, not gonna be Robin. And they've had 'contingency mothers' in every season so far, in case they had to wrap things up quickly. It could have been Vittoria, Stella, Rachel Bilson's character, and a couple others I'm sure I'm blanking on at the moment. Early seasons, they were on the cancellation bubble, so they always had an emergency 'out' if they needed to wrap up.
WOULD be nice if they moved things along, although the yellow umbrellas and ted's finally looking ready to move on hint that maybe we're moving again. Would be nice if he met her at the end of this season, and they actually spent the final season introducing her, ending with Ted getting married off and wrapping up the story that way.
Also, found it funny that when he was trying to take the mother to the movies this episode, the movie they couldn't get into was The Wedding Bride III, apparently the 3rd movie in the series about his failed engagement to Stella
One thing this series does really well is the callback jokes and continuity. Most episodes have tons of things like that thrown in there, really ties things together nicely. The blue horn being chained to the wall in the restuarant was a nice touch! Yellow umbrella was obvious. Sometimes it's little things, like when Marshall was stuck on the roof this year because the kid trapped him up there to have a party, and they showed the kid eating a sandwich. Go back a bit, and sandwhich is code in the story for the characters smoking weed. took a few minutes to figure that out, but then you get a good laugh out of it...