I understand that OOO needs to make money but the players also want to have fun with the game. There are a couple simple things that could be done which would accomplish both.

1. Let people BUY trinket and weapon slots instead of renting them. For me, there is zero incentive to pay a couple hundred energy just to rent a slot for a month. However, if you let me purchase a permanent weapon slot, I would most certainly do that. That works out well for OOO as well, as the more weapons you can equip, the more weapons people will buy and the more CE will be sold to craft those weapons. You could price it something like:
-2 weapon slots free like the current system
-3rd permanent weapon (or trinket) slot 500CE
-4th permanent weapon (or trinket) slot 750CE
-5th permanent weapon slot 1000CE
-6th permanent weapon slot 2000CE
-7th permanent weapon slot 2000CE

Of course, for this to really work you would need to remove the binding 4 and 5 star items so that crafters can make and sell them again but I would suspect that the majority of the players would buy several permanent weapon and trinket slots. It seems like OOO doesn't understand that it works in everyone's favor for people to use a lot of CE and I would suspect that the 4 and 5 star crafting market was a HUGE part of that.

This would also create more 'fun' for the players. Are you stuck in a T3 graveyard? Good thing you have your 6th and 7th slots filled with 5 star undead damaging weapons just for this occasion. People would probably stop avoiding danger rooms as they would have better suited weapons regardless of what mobs popped up. Furthermore, just think of high end players running around with 7 5* items they can use. That is a lot of CE being spent.

2. In dungeon 'dressing rooms'. Again, this is something that could be purchased for CE. This item could be set on the floor and allow everyone in the party to change their equipment in the middle of a dungeon for a limited time. Again, this would encourage people to have a lot of different weapons and armors so they could be effective in every situation. Just in case a 'bad' strata popped up, you could switch to shock defense armor and keep going. Again it would be a CE sink plus probably something that quite a few players would enjoy.

3. This will probably get a lot of negative comments, but don't allow people to equip 5* stuff right away. You lose if someone starts the game on day 1 with all the best equipment. That doesn't really benefit anyone. Currently you have the game set up so that you have to have 4* equipment to progress beyond a certain point. It would make logical sense to also make it so that you can't equip 5* stuff until you have beaten a certain level. Maybe you have to kill Vanaduke for the first time in 4* equipment before you can equip 5*. Maybe you have to kill the JK in 3* before you can equip 4*. Whatever you do, you should not let people equip 5* stuff before they ever run a single dungeon.

4. Encourage people to spend CE. Maybe give a bonus for people doing 15 levels in a row (which would cost at least 50CE, assuming your mist tank was full). Maybe once you do 15 levels in a row, you start to get double the crafting mats. Maybe you can only get certain tokens by running every single level in a row in one gate (level 1 through Vanaduke). Those tokens could be traded for 6* equipment.

5. You really need more content if you want players to stick around for awhile. I know it is easy for me to say but you need 2 more bosses 6 and 7* equipment, etc... The most fun part in the game right now for most people is doing boss runs, and for there to be only two bosses... That gets old quick.

That is all I have off the top of my head. I would be curious to hear what others think of my ideas.

TL;DR - don't be a tool. It took way longer for me to type then for you to read it. Just read it.