Went to a few Packers/Buccaneers games in the late 90s before they tore down the Big Sombrero. That place was always packed with Packers fans.
Accessible tickets + excuse to travel to Florida = Let’s Go!
Vanderbilt’s football stadium is also regularly taken over by fans of their more popular SEC opponents.
Jets fans are famous for outnumbering Dolphins fans in Miami. So much so that last month a player whined about it during a chat with fans:
Speaking as an ND Alum and fan, the ND administration was so embarrassed by seeing so many Nebraska fans at that game that we alumns were admonished that in the future if we sold our tickets we would be removed from the rolls and wouldn’t be able to buy tickets in the future. We alums responded, “Hey, our team wasn’t particularly good, the Nebraska fans were paying top dollar (several times the already high face value), and we’re well-educated good capitalists, so of course we sold the tickets. What do you expect?” 
Until recently, the Washington Nationals home games were an opportunity to see how many transplants there are from whatever area the visiting team hails from.
But the most striking example of this phenomenon I have ever seen was when I went to a Montreal Expos game in 2003. The whole stadium was taken over by 800 people who didn’t seem to be baseball fans at all. It was really a bummer for the 100 or so baseball fans who did attend.
Packer fans are known for traveling to away games, in part because home tickets at Lambeau are hard to come by.
When the Packers were in the NFC Central with the Bucs (and played at Tampa every year), it became a point of embarrassment for the Bucs. In more recent years, Packer fans have effectively taken over the stadiums at St. Louis and Arizona, especially later in the game, when the home team is losing.
Way back in the day, the UF/FSU football games in Tallahassee were this way. More blue/orange than garnet/gold.
Although not a takeover, when FSU played in South Bend, there were enough Seminoles there (probably ~5-10k) to make the stadium, if not friendly at least inclined to neutral. It didn’t help that the 'Noles smoked ND. Not much for the Irish faithful to cheer about. Great hosts though. Kind of turned me into a bit of an Irish fan.
Ditto about the Nebraska fans. When we played them in the Orange Bowl for the National Championship they were incredibly gracious. One the few times I’ve seen FSU fans fight each other (one was being a dick to some Nebraska fans on the Tri-Rail after the game and a couple of true fans stepped to correct the situation - he ended up apologizing for his behavior).
Except Kyle Field; I have yet to see anyone outnumber Aggies at home in 25 some-odd years of going to A&M football games.
Pittsburgh fans must be die-hard; this has happened to me watching the Caps play the Penguins on Washington ice and whole sea of green in the stands.
Every Cubs-Tigers game I’ve been to has been very solidly orange and blue in the stands. And the Tigers fans seem to be louder than the Cubs fans, but that’s probably because the Tigers are usually whooping the Cubs’ asses up and down Clark.
I’ve been to Cubs/Cardinals games in St. Louis and seen at least as much blue as red. Maybe not a “takeover” per se, but a run for the money anyhow.
Not sure if this really counts, but in 2008, Hurricane Ike forced the Houston Astros to move some of their home games out of town. They played two games as the “home” team against the Cubs in Milwaukee’s Miller Park, just 90 miles north of Wrigley Field. Cub fans came out in force, and the Astros got a total of one hit in the two games.
The one road hockey game I’ve been to was in the CHL a couple years ago. There were more visiting Fort Wayne fans in Dayton than there were Dayton fans. I’ve heard it comes close when Fort Wayne plays in Kalamazoo, maybe a 40/60 split in the stands.
Regularly in international rugby - Wallabies/All Blacks games in Australia can usually rely on seeing more black in the stands than gold, and the ARU often schedules Wallabies/Springboks games in Perth in order to sell out the stadium to South African expats.