First off, I know at reasonable rates it takes very long to even get to level 150 and meet requirements to turn into "Dragon Slayer".
This just came into my mind when I "found" some old dr folder with old screenshots.
So my suggestion is basically to add more dual steps/level to the game.
Reason: game will take even longer to "end".
It's not high priority of course and to make that actually useful new monster would probably also be needed etc. etc. but that's kind of the purpose of the thread.
Maybe, if there are some people who are also interested in adding new dual level to the game, it could be discussed what those should be like and what is needed for those.
So once the developers fixed all bugs, the beta is over and everything else they want do is done they could use this to decide if they want to add new duals at all and if so, how it could be done.
So I mentioned this came into my mind because of a screenshot, it's from my own test server I was working on some years ago.
I implemented a dual 6 there, simply called "Dragon Slayer II". My brother also made me a fancy icon for it (I'm not good with graphics).
I'll just post that screenshot as an idea how it may look like. However the title was supposed to be temporary only and the extra stats I gave those were probably not balanced.
edit: as I thought about it once more, I guess it seems too abstract for most since d5 is so rare already.
Therefore another way could be to keep d5 "Dragon Slayer" as last rank but change max level along with tactics to something higher once a lot of people managed to get dual on non-beta server.
It doesn't necessarily has to be extra duals. Also of course, any opinions for not changing the max level at all etc. are welcome as well.