While it is true that updates have been lackluster for the past... two years now? I don't think we can expect anything big and game-changing at that point in development I'm afraid. KF2 is more than five years old now and thus long past its prime. It's already incredible that the game is still getting updated you know?
Bringing every perk to level 30 wouldn't work. Not only because you'd need to come up with 20 new skills (2 per perks), but also because the HoE experts would find the game even easier. It would also be weird for the players who prestiged the game : would they get downgraded, or still be at maximum prestige but pretty much "skipping" multiple levels?
Same problem for level 35, but obviously doubled. Not to mention that "after a few years" sounds like wishful thinking at best : their probably WON'T be a few years from now on. I'm already impressed we're still getting content this year. Considering a KF3 is rumoured to be in development, I would expect it to be the end of the line.
New achievements would be neat, however. I always said that I would have loved them to add very crazy and difficult ideas to keep players on their toes. Killing four zeds with a single boomstick shot? Neat ! Completing a wave with only the C4? Can do ! Landing the killing blow on a boss using your knife? Woop woop ! Killing X amount of zeds using their own weapons ( Killing Hans 5 times with a Mkb42, Killing 20 husks with the Husk Launcher,...) ? Damn !
Alas, I believe we'll be stuck with new maps and new guns for the most part. And I agree : I usually come back for a single weekend every three months, then go on to play other things...
I would love for Tripwire to stop making updates for a while and concentrate full force on KF3. Then after a year or two... BOOM. They announce the third game and to make us wait for the release, they launch a big, final update for KF2. Packed with content.