We'll give you the latest details about what you can expect in Town Hall 18 for Clash of Clans.

Clash of Clans is an ever-evolving game, where regular updates introduce new additions that change the way it's played and how players approach their strategy.
The biggest type of update that leaves most players on the edge of their seats is new Town Hall levels. Town Hall 17 is currently the maximum level, and all eyes are now set on the next level, Town Hall 18.
Supercell is often tight-lipped about the details of upcoming updates. The community only ever receives information about it shortly before it's released. However, with Town Hall 18, there are small hints of information that give us an idea of what's to come.
Town Hall 18 May Be Released at the End of 2025

Supercell has previously confirmed that it will "roughly be a 12-month cycle between new Town Hall levels." Since Town Hall 17 was released on November 25, 2024, Town Hall 18 may be released towards the end of 2025. There is also a potential for the new Town Hall level to be introduced earlier than that or during the first few months of 2026.
You can use the Hammer Jam event as a good indicator. How? This special event usually starts a week before a new Town Hall level is released. During this time, players will be given a 50% discount on all Home Village Buildings and upgrade costs. This allows them to catch up with all the potential upgrades that are on their way.
What New Troops & Buildings Will Be Added?
Brand-new Troops and Buildings will undoubtedly be added in this Town Hall update. However, the exact details of how each one will manifest are still unclear.

For Troops, an Elixir Troop will most likely be released in Town Hall 18. We say this because a Dark Elixir Troop, known as Furnace, was introduced to Clash of Clans during the Clash Anytime update in March earlier this year. Supercell most likely won't release two Dark Elixir Troops in a row.
As for Buildings, this is a bit of a tricky one to predict. In the past, Supercell voiced their concerns about their "soft limit" of 100 Buildings in the Home Village. As shown in previous updates, they're drastically reducing the overload of structures. We saw the removal of the extra Barracks, the reduction in size of Hero Altars, and the introduction of the Defense Merging mechanic.
With all that said, we could see a limited number of Buildings (be it a Base Defense or another structure) added, but if they don't, then there very well may be an expansion of the Merging mechanic.
Will A New Hero Be Introduced in Town Hall 18?

In a Reddit AMA held shortly before the release of Town Hall 15, Supercell answered a question about the decision-making process that goes into deciding whether to introduce a new Hero or not. They said:
We would like to add new Heroes in the future but we also have to keep "simplicity" in mind on how many things need to be deployed for each attack. [...] Does it fill a specific niche? We've got a tank, we've got two ranged Heroes, and a flying one that is also a support one. Do we want a new Hero to be purely air based? Is that even necessary or does it fit within the game mechanics to make it enjoyable? Couple that with the additional factors of base design, we don't want to add too many Heroes too quickly.
Fast forward to today, and we can see that this answer is still relevant in the current meta. Supercell does not want to add new Heroes to Clash of Clans for the sake of adding a new one.
A new one will only be added if a game-changing addition or mechanic is introduced alongside the update. If that's the case for Town Hall 18, then there is a very high chance that a new Hero will be on its way.