I grinded my way through eureka and it was the absolute worst experience I have had with this game. Eureka was garbage content because it didn't play to the dev team's strengths. Simply put, they suck at making open world content. This game isn't designed for it and they have no idea how to make it work. What they are good at however, is designing very controlled encounters, which is to say, boss fights. To that end, I'd love to see something like a boss rush mode. I imagine it'd work something like this:

You're trying to upgrade your relic by imprinting it with the memories of your past battles (not unlike what you did during the last step of the heavensward relic quest). To that end, you use those memory crystals from the current relic quest to create a simulation of every boss fight in the game. When you go into the simulation you get your relic weapon and a special set of simulation armor. Both would be upgradeable.

When you enter the sim it puts you up against a random boss. You'd be given 60-90 minutes to clear as many boss fights as possible. There would be multiple queues for this, each of which pull from different pools of boss fights and offer more or less exp depending on which pool its pulling from. Going in order from least rewarding to most rewarding you'd have;

1) A standard 4 man queue that pulls boss fights from every dungeon in the game.
2) A standard 8 man queue that pulls from every normal/hard mode primal.
3) An extreme 8 man queue that pulls from every extreme primal in the game and finally,
4) An 8 man savage queue that pulls from every savage fight in the game.

You wouldn't be leveled down or synced when you go into these fights, you'd have your full level 80 kits, but the bosses would have their numbers brought up to compensate. No new mechanics or anything, this system would already expect you to know the mechanics for every fight in the game so adding new wrinkles into that would be massive overkill, just give them bigger numbers.

Every time you fight and kill a boss, you get some exp which raises your sim level up to a max of say, 100 (or even higher if you want to make it grindier, the max number of levels is mostly arbitrary). The higher your sim level, the faster and easier you can take down bosses and the more bosses you can clear within the time limit. At certain level break points (like say, every 25% of whatever the max level is) your gear would go up a number of ilevels in the real world.

In order to encourage players to deal with some of the harder bosses and not just bail the moment they get say, T9 or pepsi man in the savage roulette, boss rewards would be scaled up or down depending on the “difficulty tier” of the fight. The fights would be automatically assigned to a tier based on data collected from players including the average time spent in the fight, the number of wipes the average team suffers during the course of the fight and the number of disbands that occur whenever a team runs into a specific boss.

This game has a ton of fantastic boss fights that are mostly rotting away in dungeons that people don't bother playing any more. I'd hope a system like this would be easy enough to make since it wouldn't really require too many new assets, it'd just plop players down in the boss room that already exists and it would get some use out of those fights that are just sitting there, ignored and unused.