The art style and some character designs of Mega Man X8 were changed to make the characters look more human. This is (for now) the last game in the series... which bridges into the Mega Man Zero series, where Reploids are basically indistinguishable from humans.
Usually your current Hunter is replaced by the backup if they are defeated, but they aren't when you fall into spikes or pits. Considering your partner spawns in the very same place the first Hunter lost, spawning into the spikes and pits means they pretty much gets destroyed at the very first contact, so naturally you'll die no matter what.
In Noah's Park, Axl brazenly jokes "Better watch out, X. I might just steal your spotlight." This could be a knowing jab at Mega Man X7 for effectively replacing X with Axl for the majority of that game.
It may seem rather random that new generation Reploids have the ability to turn into copies of other Reploids... But remember what the key feature of the Reploids' template is; copying other Reploids is actually an evolution of X's variable weapon system.
This is best exemplified with Lumine's battle. What are the key attacks of his first phase? Using boss abilities, complete with it changing his color to match the weapon used. Not just that but his version is copying their Turns Red attacks. X is basically fighting a directly upgraded version of himself.
Why does Sigma suddenly deride Axl as a useless prototype after hinging his entire plan on the kid in X7? First of all, there were plenty of new generation Reploids to choose from now, so Axl wasn't all that special anymore. Plus, in X7 Axl defeated Sigma by outsmarting him, something the old guy isn't used to, and which may have fostered a grudge.
A very minor detail is the difference between the icons on boss health bars. For most of the game, every boss has a skewed yellow "Σ" all the way up to Copy Sigma. However, the real Sigma has a completely different icon that more closely resembles the wolf-inspired "Σ" logo from previous entries, signifying that he is the real deal. Then Lumine has his own emblem too: a curved angel wing in the shape of an "L", showing that he has surpassed both Sigma and the old Mavericks to form his own type of revolution.
Earthrock Trilobyte's stage seems to be located in western Canada, which in real life is home to the Burgess Shale, where many fossils of marine invertebrates, including trilobites, have been discovered.
X8 is the game that kills Sigma off for good. While this revelation seems to come out of left field, there are hints that this is going to be Sigma's final appearance:
First, the design of the Copy Sigma(s) is an updated version of Sigma's original body circa X1, while the real Sigma's design is a scrappy-looking black devil skeleton. In featuring Sigma's original form alongside his final form, the game creates a contrast between the two to provide a reminder of how far Sigma has fallen from his former glory. Sigma's shoddy appearance also provides a visual cue that he's on his last legs.
Second, Sigma finally provides clarification as to why he went Maverick in the first place. He tells Zero that it was being infected by the Maverick Virus that set him on his path (information we already knew), but he tells X that it was X's dedication to justice that first got him thinking about how imperfect the world was and why (this is new information). In other words, according to Sigma it was both the virus and ideology that made him go Maverick. This is much-needed closure on his entire motive. note Put simply: Sigma was inspired by X to think about the world's imperfections while the virus removed his inhibitions, allowing him to act on that ideology by rebelling when he otherwise would have never considered doing such a thing.
Third, Sigma talks about how the "old world" and the old generation Reploids have to be destroyed for the sake of the new world he wants to create in space. Naturally this implies that Sigma himself has to be destroyed as well since he's part of that old world too, with Lumine (the new incarnation of Sigma) replacing him as the Maverick leader.
Fourth, when the Copy Sigma contacts Hunter HQ, he tells X that they'll never have to meet again when this is over. Once facing him in Gateway, he also tells X that it's time for their final showdown. Then when the real Sigma confronts the Hunters, he tells X that this is the moment their destinies come to a head, and he muses to Zero that they are crossing paths at the end of the world. All these statements imply that this game will be X and Zero's final battle with Sigma. note Neither Sigma's statements to Axl have this sense of finality, but that can be chalked up to Sigma having no personal history with Axl as well as viewing Axl as an inferior prototype unworthy of his attention.
Fifth, Sigma doesn't vow to return upon his defeat like he usually does. In previous games, Sigma vowed to return after his defeats, with only three exceptions: X4, where he died satisfied with the results of his machinations; Xtreme 2, where he made no final statement at all; and X5, which was meant to be the final game of the series. In this game, Sigma's last words and scream of agony indicate that he did not die content, so his lack of a promise to return implies he already knew he couldn't.
He doesn't even have a One-Winged Angel form this time, dying much earlier in the battle than usual.
Sixth, the fight took place on the moon - Lumine's base. Not only were communications jammed by Sigma, but Lumine possibly cut Sigma off from the internet because he planned to exterminate all old Generation Reploids. Also, the final level is very sparse, consisting of basic structures compared to the completed fortresses of the previous games. Between all that and Sigma wanting to abandon Earth, there might not have been any internet access in the first place.
Still, Sigma won in this game. Despite being destroyed for apparently the last time, Sigma ensured that revolutionary Mavericks like himself would always exist. By getting his data into the new generation Reploids and leading their revolt, he planted seeds that would carry on into the future forever. No longer would he need to manipulate those such as Doppler or Repliforce to change the world - an entire race of robots was now capable of carrying on that dream for him. That's why he refers to them as his children.
Sigma's defilement of the supposedly perfect new-generation Reploids is even similar to the story of Lucifer and the Fall of Adam, where one man's temptations caused an entire people to become dangerous and mortal.
The epilogue claims there was an attempt to stop Sigma's seed, as production of new-gen Reploids was halted. However, it also says that production resumed shortly after, so Sigma still won.
Continuing into the Mega Man Zero series, Sigma's virus lingered all the way until X used the Mother Elf to erase it. Then Dr. Weil wanted to punish the Reploids for their disobedience, resulting in the Elf Wars, meaning Sigma's actions propagated even more bloodshed than he ever dreamed. Not that he would ever agree with Weil, as the two are quite opposite in their ideals on Reploids.
, where many fossils of marine invertebrates, including trilobites, have been discovered.