I'll just mention right away that I believe this is an unlikely situation due to licensing and the shuffling of characters from Fox to Disney, but here's how Deadpool could fit into Avengers: Endgame. Again, you have to keep in mind it'll take time for the X-Men characters to be shuffled, but Deadpool has always been a wild card.
This is the type of character that breaks all walls, physical and beyond. What's interesting and probably not at all connected is that at the end of Deadpool 2 he gets a time machine. Now, while it's set to bring you back and forth the whole thing is very inconsistent as to what it can actually do. I say that, as he does wild things with it that may exist within his timeline as well as outside of it.
I don't want to get into the finer details, but this has the potential to perhaps throw him into the regular Marvel Cinematic Universe. It could come in the form of a cameo, or an appearance of some degree though it would be neat to see. The Avengers appear to be using time to their advantage so anything could go on. They'll be testing the waters to see how Deadpool as a regular character works with a lower rating and that in turn could make him possibly more appropriate for the general viewing audience that consumes the usual MCU films.
There have also been talks about getting the X-Men characters integrated someday and Deadpool is a good start as it's easy to keep the character while leaving everything else behind. You can pick and choose what you'd like from him to solidify as canon and forget the rest. It would be neat to see Deadpool in Avengers: Endgame and that time machine he has is the potential method for how he could fit in. You can read our review of Infinity War below and check out the Marvel Cinematic Universe hub for additional coverage of the franchise.