It's a MMO. You can't have continued progression or the gap between long time players and new players creates an unbalanced experience.
XP points exist to get the player to level cap. There's no reason for there to be continued reward once there. XP also gets replaced by things like tomestones in some content once you've reached level cap so there's your equivalent of a treasure coffer.
Regarding things like teleport costs and speed increases: if you put a cap on those, then you've once again stopped progression and you've recreated the original issue you want addressed. People will start asking for more rewards to replace the ones they can no longer earn. Where does it end? Do those bonuses get taken away once level cap is raised so they can become meaningful again once the player has reached the new level cap?
Your idea for using merit points to buy unique items sounds an awful lot like the Achievement Certificate system already in place. The only current drawback to it is that it primarily offers the old veteran rewards with few new rewards being added. SE really needs to expand what rewards are being offered.
Regarding using gear to impact how jobs function, just a flat out no as far as I'm concerned. Once you start moving job function from character progression to gear progression, you've just killed the player's investment in their character because their character no longer can progress and they rapidly lose interest in the game. Go play WoW and see what a disaster that turns into.
No. An MMO is about playing with other people. It's not about endless progression.
If you feel like you've hit the limit for what you can do with your character, create an alt and start the process over again. That used to work very well for WoW because it kept characters fairly close in progression regardless of whether a player had 6 hours a week to play or 30. Then Blizzard caved into the part of the player base that wanted endless progression. People felt overwhelmed. They felt tied to a single character and every moment not spent playing that character was the character failing to progress. It went from being a game to being a never ending job.
Go play WoW if you want endless progression.