In the fanfic, Bardock might be filled with pride. He could be thinking how his son has grown into such a powerful warrior. After all, Goku has come a long way from being a little Saiyan baby sent to Earth. Watching Goku fight Buu, Bardock might see all the potential he always knew Goku had, finally being fully unleashed.
Well, Bardock could be feeling a mix of emotions. He might be worried at times, especially when Buu seems to have the upper hand. But he also has faith in Goku. Remember, Bardock was a tough Saiyan himself. So, he may be analyzing Goku's every move, thinking about how he could have fought Buu differently or how Goku is using Saiyan techniques in new and creative ways.
In the fanfiction, Bardock might be in a state of awe. Goku's fight against Buu is no small feat. Bardock would likely note Goku's ability to keep getting back up, no matter how hard Buu hits him. He might also be thinking about the power levels involved. In his time, the power levels he knew were much lower. Now, seeing Goku take on a foe as powerful as Buu, he'd realize just how much the Saiyan race has evolved, especially with Goku as a prime example.
His presence can add an emotional depth. It could be a moment of connection between father and son, even if Goku doesn't know Bardock is there. It might also serve as a reminder of Goku's Saiyan heritage.
It could potentially influence Goku's fighting style. Bardock was a fierce warrior with his own set of techniques. Goku may subconsciously incorporate some of Bardock's fighting spirit or even mimic some of his moves. Also, Bardock's presence could be a source of inspiration for Goku, making him come up with new strategies on the fly during the fight against Buu in the fanfic.
In a 'Bardock meets Goku' fanfic, the plot might involve Bardock being brought back to life by the Dragon Balls. He then sets out to find Goku. When they meet, Goku is initially confused but then realizes it's his father. They could then train together and Bardock could teach Goku some long - lost Saiyan techniques. It could also be a more emotional meeting where they talk about Goku's past and his mother.
One possible plot could be Bardock's adventures before Goku was born. Maybe it could show how he dealt with the Saiyan politics and his growing concerns about Frieza's rule. This could set the stage for Goku's future battles as his son.
He might be really confused at first. Since the abridged fanfic often has a lot of comical and sometimes out - of - character interpretations. He'd probably be scratching his head at some of the jokes made about himself and his friends.
One possible interaction is that Bardock could spar with Goku and Gohan. He'd test their limits and see how much they've learned about fighting. Another could be Bardock sharing stories of Planet Vegeta with them, which would be really interesting for Goku and Gohan to hear.
It could be a story of redemption for Raditz. He starts off as a bit of a villain, but through interactions with Bardock (either as a memory or some kind of spirit guidance) and Goku's influence, he changes his ways. They could all be training together on Planet Vegeta before its destruction, with Bardock teaching them valuable Saiyan combat techniques and life lessons that shape them into the characters we know.
One idea could be to focus on Bardock's perspective before his fateful battle. Maybe explore his thoughts about his son Goku being sent to Earth and his own impending doom. He could have some premonitions about Goku's future role in saving the universe.
I'm not sure of a specific highly recommended 'Bardock finds Goku' fanfic off the top of my head. But generally, fanfics with a high number of reviews and positive ratings are likely to be good. So when you search on FanFiction.net or AO3, sort the results by the number of reviews or ratings. Check out the ones that seem popular and read the summaries to see if they match what you're looking for in terms of Bardock finding Goku.