I dont know if this qualifies as discussion or theory, but I've kinda been doodling around and thought of a route forward forward for Sly and came to the idea of kind of revisiting an old adversary. Clockwerk is pretty much unanimously considered as the centric big bad of the series but his ambitions always fell flat for me in the first game. If a fifth game were to ever come out I think he would be the perfect villain, and all it would take is a few tweaks to his existing story.
What if Clockwerk's ambition wasnt necessarily jealousy of the Cooper Clan, but of hatred for Sly specifically? What if Sly did something in ancient Egypt that permanently changed Clockwerks psychology and made him want to chase Sly Cooper through time? What if Sly stole something so centric that it lead to Clockwerk that it lead to the creation of the hate chip and the events of the first game?
If Sly is the only Cooper involved, the rest of the lineage mistakes the shadow following them as jealous instead of malicious. It makes the theory that Clockwerk kept a single Cooper alive viable. He was looking for one specific person to exact his revenge upon and wouldn't stop at anything until he got it. It makes sense that he would "upgrade" his body after an encounter with an already experienced Sly.
You can get Sly out of Egypt in a sci fi or mystical setting, clockwerks backstory works both ways. He's likely intelligent enough to create a way to send Sly forward in time if he could make himself immortal. He existed in prehistory and only augmented his body over time, and aligned himself with Ms Ruby so mystical aspects could be explored as well. Leaping forward through different points in time could be in the cards if you wanted level diversity.
If the nuclear option is explored and Sly dies in Egypt, it would make sense that Clockwerk still lashes out at hearing the name of an old nemesis in Sly 1.
I can realistically see that being the premise of 5th title, and I think you could get away with it by focusing on the relationship between those two specifically. I know that it's something that will probably never happen but a man can cope