I really want to see a fairly serious attempt at this pair that would keep them roughly in character and do the concept enough justice for all the Taiga route fans out there.
The way I can see something like this deveolping is that Taiga gets curious about what Shirous always up too in that shed, maybe jumping to the conclusion that it could be either perverted or dangerous. So she decideds to peak, if it's dangerous she'll stop it but if it's just him discovering puberty she'll leave him at that (god knows it would be embarassing both if she let slip she saw). She just sees him meditating very hard on a metal bar. She falls for the mild BARness Shirou's putting out and continues to watch until he starts to seem like he's in very bad pain, before she can rush to his side the pipe gets warped to fuck because of the failed Reinforcment. And so Taiga confronts him on the obvious magic and won't take any excuses until she gets the truth.
Or Taiga could have witness the BAR when Shiro was a little younger, around 14 or so, and absently noted how her little Shiro has grown into quite a handsome and strong young man. She would chide herself on becoming some kind of couger and try to dismiss the thought but the seeds would be planted and might start to think about how the age diffence would be too big when he grows up. And so Taiga half serously and mostly uncountously starts to mold Shiro into her future man.
And of course the obligatory idea ripped from H manga. Taiga got dumped and turned to alchohal and shiro for comfort. One thing leads to another and the nights ends with them in bed sweaty, tired, and satisfied.