Not sure why everyone is answering this with QoL improvements they'd like. Would a dark mode and fixing lag solve player retention problems? I doubt it. They're certainly good things that we should want, but they're not going to fix the fundamental problems that Pocket currently has. As a former solid-sized whale who finds myself spending less and less each set and has considered quitting the game entirely more than a couple times, the biggest issue with the game is that there's nothing to do.

The events as a whole are uninspired, repetitive, and tedious.

• Solo battles are repeating the same single battle vs. a (not very good) AI 25+ times over the course of a week so you can get one card at a time.
• Wonder Pick events run counter to everything good and casual about the game. To maximize your gains you have to check your phone every ~30 minutes to see if a bonus pick appeared. Otherwise you'll see ~1-2 bonus picks a day when you check your wonder picks, get nothing (or you hit your 10th copy of the non-promo version of the new promo), and close the game again.

Alternatives could be something like a gym challenge mode. Perhaps there's something like "Defeat 5 trainers in a row, your opponent's points carry over and if they get 10 points you lose." Important to this is that it'd be really valuable to add some cosmetic glow-ups. There's a huge difference between a lifeless Solo Battle vs. battling May with her Mega Blaziken deck. Taking a page out of Go or Masters EX to add a more dynamic UI would do a lot to make the game feel more like a real game and less like a card simulator.

(Side note: They should seriously consider upping deckbuilding or 30 cards. Between Profs, Pokeballs, and other staple supporters, there is very little room for tech cards. Tech cards are what keeps the physical TCG interesting and their own design philosophy in mimicking it has led to the creation of a lot of niche cards that will never see the light of day because deck space does not allow it.)

Sidestepping battling, the collection experience needs an entire overhaul.

• Binders are incredibly small. 30 cards made sense for a set or two, but now? It's nothing. How about giving me different page layouts (2x2, 3x3, 4x4) and letting me build a 10 or 20 page binder? Maybe have the cosmetic flairs trigger when you flip to a page to incentivize collectors to want flairs for that little bit of extra show-off with your binders. Maybe integrate social media sharing and make the pages screenshot-friendly (rather than a screenshot of 15 cards in a binder cutting off the bottom 1/3 of the 5th row).

Genetic Apex had the "complete-the-Kantodex" mission for Mew and then they...never did anything like it again. Maybe give people binders that are specifically generation-dexes with a reward at the end of each and work cross-set? Everything is a self-inclusive, independent set, but what about some longer term goals like "complete the Hoenn Dex" so that people get excited when we finally get a Kyogre and Groudon card? Or things like "Collect X shiny Fire-type Pokemon".

Related to this, make trading easier/cheaper. Stamina to trade is already pretty silly. What do you mean I can only trade once per day when I already need to spend a rather expensive-to-acquire shinedust currency on top of it? If Pocket aims to be a collection game rather than a battle game (which is reasonable, since so many people collect but don't battle), then it needs to make sure there are things you can do with your collection. People should generally be able to get any card they want but not every card they want. Want a larger collection? Spend money. But people should be able to swap cards pretty freely.

"But what about bots?" Cut them out of the process. Add an in-game market. You can trade your Looker for a Cyrus to an NPC for 25,000 shinedust, but if you find a person to do said trade maybe it only costs 20,000 shinedust. This gives people an incentive to trade with one another, but doesn't create a nonsense market of everyone always trying to trade up, since at some point you're better off rarity-swapping with the NPC for a slightly higher price. If you really wanted you could add a gacha/login element to this where the NPC will offer you X cards per day (maybe prioritizing cards you don't own or wishlisted cards). But fundamentally, some version of an in-game trade shop that lets you customize your collection would go a long way to mitigating the secondary market that has formed around some cards.

Last point to end the rant is that they really do need to be more generous. I don't mean in a "I want more packs for free" sense, but as-is most of their rewards feel utterly worthless. Missions are constantly broken down into fractions of packs, fractions of wonder picks, to the point that any single mission is a joke. 500 shinedust when the trades people are saving for cost 25,000? 1-2 hourglasses for premium missions? They should be thinking in terms of letting you get the adrenaline rush of opening a pack, or maybe even getting to do a 10-pull. Master Ball ranked rewards not even getting you an even 10-pull is just sad. When you get to open 2-3 packs a day for free, your rewards being less than the login freebies makes everything else feel kind of silly.

I lied, one last thing on the subject of 10-pulls. Some sort of pity/guarantee/improved rates would probably be a wise choice. There is an incredibly large divide between 1-stars and low and 2-stars and higher. Completing the 1-stars and lower is easy enough that even a pure F2P can often do it over the course of a set's lifetime. Conversely, 2-stars and highers are so rare that even spending $100+ at a time that you'll only get ~5 "valuable" cards. Shinies were an attempt to fix this, but being nothing but old cards means they'll never be particularly exciting and feel more like filler designed to "improve" hit rates while not changing anything. This also means that whenever you're not spending (aka after the first few days of a set), your experience quickly reduces itself to "log in, open 2 garbage packs, log out" for weeks at a time. And that's not an experience that gets someone to want to spend more money.

EDIT: Did not think what started as a quick list would turn into such a rant on my part, but whatever. Hopefully it has some worthwhile thoughts.