Ahead of its season 2 finale, Young Sheldon announced that it will feature the younger iterations of the rest of the main cast of The Big Bang Theory. CBS has yet to reveal how they're planning on executing this, but considering the logistics, this is virtually impossible to pull through unless the prequel spinoff wants to risk creating a plot hole: Sheldon hadn't met Leonard, Howard, Raj et al that early. The series is already criticized for rewriting established TBBT canon for their own benefit, and they can't risk blatantly doing that just to tie the show to its parent season. But there might be one creative way to pull this off in a way that doesn't break the shows continuity.
Debuting in 2017, Young Sheldon chronicles the life of Sheldon Cooper while he was child living with his family in Galveston, Texas. The series features the whole Cooper clan, including mother Mary and Meemaw. The spin-off had very few narrative links to The Big Bang Theory in season 1, but this year, CBS executed a one time-only-crossover between the series with George Sr. and a young Georgie appearing in the long-running sitcom through the discovery of Sheldon's old homemade video. But with The Big Bang Theory wrapping up its 12-year run the same day as Young Sheldon caps off its second season, the spinoff is capitalizing on the buzz surrounding the series finale with a bit that would pay homage to its principal show.
With Young Sheldon primarily set in Texas with no indication that it'll do a road trip across the country (and even India), how can the show feature young Penny, Leonard, Howard, Raj, Amy and Bernadette? The Big Bang Theory's Nobel Prize for Physics subplot may actually be the key in making all this happen.
As the only active overarching The Big Bang Theory plot going to its finale, it's unsurprising that a major chunk of the upcoming one-hour special finale will be devoted in finding out whether or not Amy and Sheldon will win the Nobel for their work on Super Asymmetry. Interestingly, this has a direct parallel to the season 2 wrap-up of Young Sheldon with the boy genius organizing a party to celebrate the Nobel Prize announcement in 1990. So it's safe to say that the connecting tissue for the the two shows will be the spectacle of the Nobel, only at different times. Chances are that the present-day cast will all be huddled together awaiting the announcement alongside the Coopers, and regardless of the decision, they're going to stick together to either celebrate or commiserate.
Young Sheldon might retrace what the other characters were doing at the time of Sheldon's gathering, allowing all of them to appear in the show but without any contact with each other - this way, CBS is able to persevere their continuity across the two shows. So we see young Leonard in New Jersey (possibly with a cameo for his mom Beverly), young Penny in Nebraska, young Amy, Bernadette and Howard (maybe with his mom and dad) in different parts of California, and young Raj in India to find out what they were doing in 1990. That would link the shows together, provide the desired amount of nostalgia, but not break The Big Bang Theory's continuity.
Aside from capitalizing on the massive buzz from The Big Bang Theory finale, this subplot can also serve as a litmus test for any potential spinoffs like Young Sheldon in the future. CBS is clearly looking to take advantage of the franchise's popularity but there's no concrete plan for another offshoot. Maybe they're waiting on fans' reaction of any of the kids playing the younger iterations of other characters before giving the greenlight.
The Big Bang Theory series finale and the Young Sheldon season 2 finale air May 16 on CBS