PB2 has become an awesome game with a huge replay value provided by the infinite possibilities of custom maps created by the community, or better, PB2 has become not a game, but an awesome plataform for games over the years, where it's own community creativity may flourish, this along with the accessibility granted by the non-profit policy of the creator (Eric Gurt) are what made (and still makes) this game so special. But when comparing to other games i loved, PB2 was always lacking in some key aspects which kept me from getting actually hooked, and feeling motivated to invest more time in the game.
As i said, the infinite possibilities given by the ALE (Advanced Level Editor) motivate me to play and create unique maps as i see the huge potential it has as a plataform for games, the big problemn is i forsee much of this potential being wasted as a lot of the game gems and masterpieces, only really reach their potential when you have a close to full match of players who have any idea of what they are doing, and PB2 at it's current state cannot provide that for most of the time ('PB2' Discord Servers also can't right now). This kills my motivation to keep playing and creating maps, and limits PB2 to a merely casual game which is much much less than what it can really be with the ALE.
A big portion of this problemn, i believe, would be naturally fixed by a substantial increase of the player count possibly brought by a future PB3 release (i also believe an effort of the community to advertise the game and of the developer to bring the game to plataforms such as steam would help to make this happen), but i will give attention to 2 aspects which could be improved independently of the player count, and that would also help to maintain the players interested for a longer time.
1. Social Aspect
When playing any game, specially cooperative/competitive ones, interacting with friends and other players can make the experience much better and deeper, be it discussing strategies, aspects of the game or just having a good time. It also makes it much easier to fill lobbies with reliable players, and therefore appreciate more complex maps. The PB2 in game chat helps a little to bring this kind of experience, but in a very handicapped way.
How it could be Improved?
a) Friends/Invite System
PB2 friends system always seemed merely cosmetical instead of functional, besides just seeing who is online, players should be able to easily see which friend is playing which match, be able to easily invite, be invited and join friend's sessions, and this without having to refresh the page.
b) Communication - Voice chat
PB3 should support in-game voice chat and the use of headsets as it is a much superior way of communicating with friends and teammates over the normal typing chat, it needs to be easily accessible so anyone can plug in a headset (or use laptop's built in microphone) to talk in a match, mute others etc. Accessibility is key, having to use secondary software as skype or discord should not be the case.
2. Grinding Aspect
Having long terms goals that require lots of effort to achieve, or are simply challenging, and being able to show off these achievements to your friends and other players is a nice way to encourage players to invest more time into the game and get actually hooked by it. We've had a sip of it with PB2's new Levelling System and with experiences as the Star Defender secret skin you earn by completing Eric Gurt's map, also with things as winner's lists in saw/traps maps provided by the custom map developers themselves, i think these kind of gimmicks should be expanded in PB3. Things as simple as winner's lists by themselves made me invest much more time learning and playing certain maps just in hope of completing them and being a part of the list, i can only dream how wholesome it would be to cooperate with friends while voice-chatting to create strategies to be on top of leaderboards of some very awesome and complex maps...
How it could be Improved?
a) Competitive play (Approved Maps)
We've already had great grinding incetives in this area all over PB2 lifecycle with the Ranking system and the feature of having your achievements immortalized in our player's profile page (maybe having something more visual to know someone reached legend of PB2 status in-game would be nice, like the player nickname apperaing in a different colour). The only problemns in this area were the connection issue, that discourages (or straight out exclude) lots of players like me from that kind of competition, and the decline of player base that makes approved matches be kinda empty nowaydays. However i believe these 2 problemns will be naturally fixed as PB3 will probaly bring more players, and having more players would also allow Eric to get more servers and tackle the connection issue, so i won't extend over that.
b) Rewards (Campaign and Custom maps)
For the Campaign and it's related achievements i think just like the star defender map, special rewards like secret skins should be given, when you complete the campaign, complete some harder achievements, one for when you complete all the achievements, always thoguht that kind of incentive was lacking to encourage people to actually care about the 'achievements' list of PB2.
For custom maps, maybe giving the map makers more control over XP rewards for achieveing certain things in the map, over statistics that can be collected over multiple games about who played the map to be able to make more in depth leaderboards, give special rewards that are permanent in the map for specific players and not only for one match. That already exists in PB2 and is possible but very complicated to do (you have to use Web API related triggers i believe, or do it manually by asking players to DM you with pictures, video, etc), so maybe make it more accessible for map makers so they can make much more than just winner lists... that may be too much trouble to implement, so i guess we may just have to learn to use the Web API thingy better.
One last and more personal wish as a map maker, is to make voting for maps more accessible as loads of players who could vote in our maps just can't, because of not really wanting to play approved maps to get the necessary kills to vote, even if they are far from being 'new players'. That would encourage map makers a little bit more, as it is very frustrating to see loads of people enjoying your map and they can't rate it because half of them are guests, the other half doesn't play approved matches, and it ends up being very rare to even get players which can rate your map. I have a topic in this forum dedicated solely to this specific issue (TL DR: use the levelling system as an alternative to allow players to vote, instead of only kills in approved maps).