*WARNING* Wall of Text Approaching *WARNING*
I have actually been having this debate in my head (its normal for me...) for quite some time now. To make just one competitively good CPU would take way too much work for it to be worth it. I have heard that the maker of the CPU AI's is actually just one person, so shoutouts to that guy (and the rest of the PMBR, cuz y'know, this mod is amazing.).
Back to CPU's though. Making CPU's actually have mixups, mindgames, and other human attributes would be so unrealistic (not that anyone was saying that...), however, I think with a little work they could still be a nice supplement to training with a human.
1) Tech Chasing
This is an acquired skill over many hours of practice, yet some characters rely on it heavily in bad matchups (like Falcon Spacies, Ganondorf Everybody, etc). I think this would be a relatively easy to implement, whether it be in the AI or another option in training mode (which is preferable in my mind). This is one of those things that seems to fit a CPU's purpose because it doesn't rely too much on option selects because there is only 8ish options (vs the many many different moves you can use to counter approaches, including non moves like WD Back and CC DSmash).
2) Combo Training
As said prior in this thread many times over, CPU's act very weird in the neutral game and it is where you pick up your bad habits from playing them. I actually do not know how difficult this one would be (or if it is already done, I can't read DI very well yet, except for throws), but to have the CPU's DI correctly/mix up their DI to keep you on your toes. Even if this is just for a couple of characters, this would be extremely beneficial. I remember a thread going up recently that was trying to catalog the correct DI for every move in the game. If they could use some of that info, it would go a long way to helping with practicing/testing comboes/chaingrabs by yourself (I got desperate and tried to see how long the SoPo chaingrab went using my foot for the opponent's DI, it didn't work very well). If this ends up too difficult/not worth the time (or in addition to *fingers crossed*), they could maybe add a DI indicator in training mode so you can get the feel for where they go with different DI.
3) Shield Pressure
This is the final request for CPU's. Another situation where the CPU would have less options so it would be easier to deal with. Have it as an option in Training mode. It would probably work like this: When on this mode, they would hold shield (probably mod it so the shield doesn't start shrinking until contact, or they don't hold shield until you get close), then they would try to escape their shield after the first hit, via spot dodge, roll, wavedash Up-B, jump+aerial, etc. This is good for seeing the timing of certain shield pressure and blocking escapes.
As you may have noticed, these are all 3 defensive tools a CPU has. Offensive and Neutral game introduces too many variables and frankly too much work. These defensive situations would be easier and more beneficial, making your alone practice more then just tech skill (that's mostly what I do now, until I get frustrated and move to that 25th secondary...). I am intrigued by the shadowboxing, so if anybody can scour for a link (I probably will eventually...), it will be much appreciated. From what I have read, it appears more beneficial once you have gotten good since you actually know the situations you need to get good at and how to address them. Thank you again PMBR, you truly have done amazing on this game, no matter how much people complain. I am not demanding these changes, these are merely suggestions that I think would selfishly help me and the other scrubs out there with small scenes/too lazy to get wifi working/too timid to go find people to play/etc. Again, thank you for reading this wall of text, thank you for making the game, thanking you for bearing with my over usage of parenthesis, and thanks you Smash community for being so awesome.
TL;DR I think CPU's could be added to with a bit of work to be made useful for some parts of solo practice.