A recently released piece of Shang-Chi and the Legend of the Ten Rings concept art features Deadpool and Proxima Midnight fighting, hinting at a possible alternate plan for the MCU's Phase 4. The piece in question shows the two characters fighting in Xialing's Golden Daggers Club, although they are not there in the final movie. The appearance of Deadpool and Proxima Midnight, though, could be remnants of a previous layout for Phase 4, with Shang-Chi possibly originally coming out during the multiverse saga.
Shang-Chi follows the titular hero as he confronts his past and attempts to take down his father and the Ten Rings organization. Although it mostly focuses on characters making their debut in the MCU, it also features some familiar faces, such as Trevor Slattery from Iron Man 3, retroactively fixing Marvel's controversial Fake Mandarin. It seems that originally the film may have featured more surprise appearances from other corners of the Marvel Comics universe, though.
During the course of Shang-Chi, the main characters arrive at the Golden Daggers Club, a fight club where superpowered individuals come to brawl. In the final cut, Wong and Abomination are seen fighting there, which is fun in its own right. Although there are lots of potential characters who could have been seen in the ring, choosing Deadpool and Proxima Midnight specifically for this concept art has some major implications. For these characters to appear in Shang-Chi, they would have had to come through the multiverse, meaning that Shang-Chi and the rest of Phase 4 may have originally been laid out very differently.
Firstly, Proxima Midnight, a member of the Black Order, is dead in the main MCU universe. Not only is she killed by Wanda Maximoff in Infinity War, but the 2014 version of Midnight also dies when Tony Stark snaps in Endgame. So, barring any more time traveling shenanigans, for Midnight to have appeared in Shang-Chi she would've had to be a variant from the multiverse. Deadpool also would have to be a result of the multiverse, as the character is not part of the MCU. The only way he could have appeared is if the multiverse pulled him from Fox's X-Men universe to the MCU. While the multiverse has been mentioned across several MCU Phase 4 movies and shows, it seems to have not been actually opened until Spider-Man: No Way Home, which takes place after the events of Shang-Chi. The only way that the multiverse could be open and allow characters to cross worlds during the events of Shang-Chi would be for the film to take place later in the timeline, after No Way Home and potentially Doctor Strange in the Multiverse of Madness. This is a massive shift in the layout of Phase 4, but it also teases another possibility beyond just the multiverse being open.
If Deadpool were to appear in Shang-Chi, that would mean that mutants would have to have been in the MCU before the film's release. It is already confirmed that the X-Men will be in Phase 4, but Shang-Chi takes place relatively early in the phase. So, either Shang-Chi was meant to be released way layer than it actually was, or mutants were meant to be introduced way earlier. It is unlikely that mutants would make their debut in Shang-Chi, so if this speculation is correct, then a larger movie featuring mutants would have originally come out before Shang-Chi. However, Phase 4 has been shifted around immensely since its original planning stages, with some projects being delayed nearly two years due to the COVID-19 pandemic. Thus, it is possible Marvel simply decided that the multiverse and mutants should appear later in Phase 4, and swapped the characters out with Wong and Abomination. It is also entirely possible that Deadpool and Proxima Midnight are just filler characters, although that is unlikely due to Marvel's incredible attention to detail. In many ways, it seems like Shang-Chi's scrapped cameos are evidence of a very different plan for Phase 4.