An idea I had was to have the players complete a side quest in order to ascend to level 20. The quest will involve their backstory and/or ambitions in some way. I was also thinking about having the players make their own level 20 quest.

what do you guys think about the idea

Isn't that just what people do normally? 

I don’t know

I mean, don't quests made by a DM usually integrate a character's backstory?

Most of the time no

At least that’s how I do it

ok

Rangers lvl 20 quest:

They should spend 1 whole year in growing a herd about your animal/bird/fish companion. At least there must be ( at the end of that spent year ) 1d8 companions. After that, one must go to a druidistic temple, and perform a ritual......... ( i'm not sure what kind of ritual... i'll navigate on webs/books for an accurate info ). After that, one must be ready to KILL an enemy, enough difficult to be worthed but, not so Impossibruh.