INTRODUCTION
(Long read ahead) At this point, alot of people have speculated about who will show up in Secret Wars, and alot of the more common names were Steve, Nat and Tony.
So i decided to come up with a sort of "experimental" theory and way to sort of rationalize how Steve could return.
Ive heard theories about how a variant Steve could show up, as well as them using the defunct time machine to de-age him, which i believe is a good way to have him return, but in this theory, I wanted to try something different, and something that could connect this more with the overall theme of the multiverse.
With that out of the way, here's the theory:

THEORY
I believe that the in Secret Wars, we will see our Steve Rogers, the same Steve that we have followed through the whole Infinity Saga. At the end of Endgame, we see that Steve got to live his life with Peggy back in the 40s, and grew up into an old man, who eventually passed on his mantle to Sam.
Well, prior to him returning to the present, there was obviously alot we didnt get to see, such as him returning the stones, getting a new shield, and the rest of his life with Peggy. I think that a way this can be explored later on in Secret Wars, would be that at the end of Steve's journey with returning the stones, he gets recruited by the TVA.
So why would the TVA recruit him? Well, as we've seen in Loki S2, the TVA has now changed its ways and instead of pruning timelines, they try and find ways to defeat Kang variants. Not only that, but in the upcoming Deadpool movie, we've seen that the TVA recruited Wade Wilson. So its not out of the question that the TVA could now chose to recruit people, instead of forcefully destroying their timelines/lives.
In the Deadpool trailer, we also saw that there is a subdivision of the TVA that helped Wade suit up and gave him his katana's. This could explain how Steve received his new shield at the end of Endgame.

The TVA could inform Steve about who theyre fighting and why they need him, for a number of reasons. Steve could decide that his fight isn't over just yet, so he chooses to help one last time. Perhaps a way he could feel the responsibility to join the fight, would be a callback to Tony's line in Endgame when he mentioned: "You mess with time, it tends to mess back". Steve could still feel like this fight isnt truly over, and that before he lives his life with Peggy, he must do whatever he can to help everyone else.
As we know, the TVA exists outside of time, so everyone in the MCU right now, (which is currently in the year 2026) is currently just experiencing the flow of time normally. Since the TVA doesnt follow that normal flow of time, the point of reference in which we see the TVA recruiting Steve would be right before secret wars is happening.

After that fight is done, Steve is allowed to finally return to his timeline where he spends the rest of his time living with Peggy. Maybe this could also explain why Steve wasnt pruned by the TVA, because by now, theyre under a different authority instead of just pruning people.
Steve then returns to the MCU in the year 2023 (post endgame) to give Sam Wilson his TVA shield and bestows the mantle of Captain America upon Sam. Since Steve is old now, he experienced the events of Secret Wars decades ago, however for everyone else in the mcu between 2023-2026, it has yet to happen.
Perhaps Steve can now live in retirement knowing what is about to happen, but not talking about it as to prevent any paradoxes or whatever.

CONCLUSION:
Anyways, this is my theory on how Steve could return for Secret Wars for one last cameo/fight. I know its a crazy theory and I dont really expect it to happen, but I thought that this could potentially be an idea as to how he could return, while tying it directly with the concept of the multiverse, as well as having the TVA be a big player in this saga.

TLDR: The TVA recruits Steve after he returns his stones, gives him a new shield, takes him to the future where secret wars is happening, he fights there, then once the fight is done, he can finally return to live his life with Peggy. This means that the old Steve we saw in Endgame has already fought in Secret Wars, and we could expect that same Steve to show up in Secret Wars while being younger.

(edit: formatting)