Despite its age, Valve has continued to support Team Fortress 2, which remains one of the most-played games on Steam. Oftentimes, the company doesn't provide much advance warning about balance changes that are coming, but in an unusual move, it's offered a preview of a number of big adjustments it has in development. The next "major update" will include balance changes that target under-used items as well as those that have been deemed either too powerful or too obvious of a choice. We don't yet have a complete list of changes, as some classes and items are still being looked at, but some classes are being addressed more than others--Scout is receiving more attention than Demoman, for instance. The proposed changes, which are not yet final, are "based on online community discussions, emails, playtime data, conversations with players of all skill ranges, and play testing."
Among the major changes are those to some of the most popular Scout items. Bonk Atomic Punch, which renders you immune to damage for a brief period of time (and prevents you from attacking), now slows you after its effect wears off. The severity of the slow is dictated by the amount of damage mitigated while Bonk is active. The change was made because Bonk currently "doesn't require enough skill or carry much of a downside."
The Sandman--the baseball and bat combo--also sees what could be considered a nerf, as it no longer temporarily disarms players. It does still slow enemies, and it should generally be easier to use--the ball doesn't have to travel as far to inflict the slow, does more damage, and regenerates more quickly.
Yet another Scout change involves The Atomizer, which allows you to triple-jump. This is being tweaked so that the bat actively has to be out in order to triple-jump; taking it out will also take longer. To compensate, it no longer deals self-damage when triple-jumping or reduces your movement speed, and airborne melee hits now deal mini-crits.
Spy also sees some significant changes. The Dead Ringer, which is used to feign death, can be frustrating to deal with when it's used repeatedly in a short span of time. Valve says it likes the effect and doesn't want to make it less interesting but is instead hoping to make it more predictable. It's doing this by removing the ability to refill the cloak meter (used to activate the feign-death effect) by collecting ammo kits or visiting a dispenser.
The Eternal Reward is being reworked completely and no longer prevents you from choosing a disguise. However, it does drain your cloak more quickly and uses a full meter to disguise yourself, though you do still obtain a "free" disguise by backstabbing an enemy. Meanwhile, the Ambassador's first shot is now less accurate, making it more difficult to headshot enemies at long distance.
Other notable changes involve the Medic's Vita-Saw (you'll now have to work to retain Ubercharge on death) and Crusader's Crossbow (the Ubercharge gain rate has been adjusted); the Heavy's various miniguns (they become accurate more quickly); the Engineer's Rescue Ranger (repairing now costs metal); and the Sniper's Darwin's Danger Shield (it no longer helps against enemy Snipers) and Razorback (you can't be overhealed).
You can check out the full list of adjustments revealed so far on the Team Fortress 2 website. There's not yet any word on when this patch is coming or what other changes or content will be included. We do know a jungle-themed update is in the works that will include community-created content, but it's unclear if these are separate updates.