Hi again.

So after thinking about the whole situation some more, I've come up with a few different options that I think strike a fair balance between what the average audience would want, while keeping new players interested. Some of the options are not interchangeable while some are. These options are 100% theoretical and you would obviously have a different perspective as to what is possible and what is not.

1)  Fleshed out season with custom framework + addon
This a new fleshed out season framework with addon for achievement and item support.

Addon:
- Additional mode to choose from, apart from normal/heroic: season 1/2
- Add custom achievements, to separate raid completion and realm firsts between season 1/2
- Add custom text to items, displaying if they are equipable/unlocked in the current season
- Add custom text to player tooltip to visualize which season they are currently in, this can be done on the server side
- Opening achievement tab of targeted player shows all season achievements

Without addon:
- Switching between season uses a command like "/season 1" "/season 2"
- When you switch seasons, you should gain/loose achievements appropriately -> achievement window only shows achievement of the current season
- Player tooltips can be modified on the server side, the season a targeted player is in can be shown there
- Opening achievement tab of targeted player shows only achievements of the player's current season (not the targeted players season)

Secondary bank:
- To solve the issue with lacking inventory space, because people would like to store their old gear for coming seasons, a secondary bank should we unlocked upon completion of a season.
- One of the bank NPCs could be converted to a secondary bank access

Technicalities:
- Switching between seasons should block if there are locked items currently equipped
- Switching between seasons should only trigger achievement gains and losses locally, so they are not displayed in the chat for anybody else watching
- Switching between seasons should block if you are inside a dungeon/raid/arena
- Additional checks should be built in for summoning or arena queues, these things should block if players are in the wrong season
- Players should get a warning in the chat on start-up if the addon is not installed
- Some LFG dungeons need to be locked/unlocked depending on the season the player is in

Server implementation details (suggestion):
- Player table needs an additional field to save which season they have currently activated
- Achievement template table needs a field that determines if the achievement is seperated by season
- Achievements table needs an additional field saving which season it is gained for
- Achievement gain/loss needs added functionality to stop sharing with other people, but still show to player
- Bank table needs to be able to store two different banks
- Raid ids table needs a new field to separate between seasons
- Items table also needs an overhaul, not going to try to write that out because i don't know the exact way this will be implemented
- Switching between seasons should activate/deactivate the correct achievements, also should support correct mechanics like blocking on locked gear and stuff pointed out up there
- Communication with addon, achievement and item querying.
- I've consulted a friend about the specifics behind this approach and an addon like this seems very manageable. Both my friend and I have no real experience with Trinitycore, so there might be things that we didn't take into consideration but we have experience with addons and there shouldn't be anything super complicated on the addon side.


Pros:
- You get to implement the seasons very closely to the original idea, while keeping everyone on the same server. Nobody gets locked out of content.
- Making an addon like this means that you have more control over any changes to the infrastructure (like seasons) as a whole without needing a custom client.

Cons:
- The addon might have undiscovered problems during it's the development and it requires new core infrastructure in order for it to work.
- If something turns out to be impossible to do, then the idea would either have to be scrapped or some kind of compromise would have to be made.


Unanswered questions in conclusion:
- What to do with Argent Tournament?
- Should characters created in season 2 be able to swap back to season 1?
- Secondary bank solution could also be substituted by giving out 36-slot-bags?
- Do we want to lock the secondary bank behind a season completion or have it available from the start, albeit a higher cost?
- We can also gradually unlock additional bag space in the secondary bank on tier completion?


Ultimately, maybe not all of the constraints listed above will make sense from your point of view, which is why I think it's important that you look at the idea objectively and try and take from it what you need.
Maybe it would only make sense if you change 30% of what was written above, it's up to you. The idea might seem overwhelming but I think even a lesser version of it would be a better implementation than just saying "your gear is not usable anymore, tough luck".
The main problem seems to be: how do you let existing players keep their character progress, while not splitting up the playerbase. If you give people the option to raid in whatever season they want, then there's nothing stopping them from never raiding with season 2 characters.
If you force everybody to raid in a certain way, then you don't have that problem anymore, but you just create other problems, like forced retirement, deleted items/achivements etc.
No matter how you do it, you will have to make a choice on what is more important, and many of the choices that you make will benefit multiple points and hurt many others.
On a very long term period, bank space and saving of gear will become a huge problem. If old gear gets unlocked as new tiers are cleared, then you can essentially save sets of gear from each tier, from each season. Fast forward 4 seasons and you can have 4 sets from each 4 seasons, assuming different specs. It's honestly insane.