Yesterday, while idly commenting on my own post, I came up with a truly astounding explanation of the "mother" situation on How I Met Your Mother. The explanation has it all: the element of surprise, romance, similarity to Friends, and most importantly, it allows you to root for Robin and Ted to be together always, even though she's not technically the "mother." In fact, if this is not what happens, I will be really, really, really disappointed.

Other possible explanations for why Ted hasn't yet met their mother:

• In an unexpected twist involving a freak accident at a routine visit to an experimental drug facility, Marshall is their mother. 
• Ted never finds anyone willing to marry him, so he adopts some of Barney's many offspring. Ted has never met their mother — UNTIL NOW.
• Same situation as my idea from yesterday, except replace Robin's name with Lily's. Robin ends up with Barney. Marshall ends up with ME [just kidding, Kyle].
• On a festive 6th-season trip to Hawaii, their plane crashes over the Pacific, hundreds of miles off course. In a mysterious hatch in the middle of the jungle, Ted finds two babies, imbuing the question of How I Met Your Mother with dark and sinister signifigance.
• UPDATE: Stolen from Mamma Mia — He doesn't know who the mother is because he's sleeping with three different women. One of them just sends him the babies and doesn't leave her name. He and his kids must solve the mystery. (This works better if Ted is a woman and has been sleeping with a bunch of dudes.)

Do you have any other ideas?