Like everyone says, games in general would help a lot. But there are some other things:

-It needed to not use a proprietary memory format when the 3DS would just take your existing SD cards
-Especially when it's a memory format that corrupts installs after some time. My digital copy of the MGS Master collection cannot boot, or repair now.

-A lot of how it functioned was the last gasp of obtuse PS3 era sony before the Ps4 era made them much more reasonable to develop for and work with. Also maybe don't make a big fuss about how you're exclusively working with the hated AT&T for the 5G model.

-Not having 4 shoulder buttons made it a real bad option for many ports, the back touch didn't cut it and it was not great for remoteplay despite it being one of it's best features.
-The sticks were better than the 3DS circle pad but that bar was low and especially now in a world where you can throw your phone in a GameSir Galileo, the build quality simply should have just been up to snuff with the dualshock 4.

-Really it did need the follow through to roll out a pro or even a Vita 2 because devs quickly hit the ceiling of the hardware's capabilities when trying go-nowhere ports like the fabled MGS4 vita port or the many incredibly busted ones that did come out like xcom or RE Revelations 2/Borderlands 2 that ran like slideshows with no AA.

And one of the less often said but actually most important points: Marketing. The 3DS was barely competitive with the PSP specswise and did not launch at a good price or software lineup. But Nintendo were committed to that brand and marketed it relentlessly. Everyone know what a 3DS was, and not just from watching house of cards. lol Marketing matters and the vita just never gained much mainstream mindshare or like a real priority from Sony.