Well I started to play on 2000 to Tibia, on that times there wasnt spoilers like now and all you have was reading the books that were on all tibia places , talk to npcs and try to find by yourselve the answer.
An example of this is POI (Pits of Inferno) before they implemented it there was a lot of books like 30-40 that Talk about a infernal place under the plains of Havoc (POH). On that time I spent a lot of time trying to find the entry to POI , I used my pick on every square of POH trying to find a secret entry, one day I found 1 entry but there was a rock blocking the way to POI , and after the update they implemented it and created the POI Quest.
Whis this I want to say that there are many quest that are still not implemented, maybe there is info on new books about new things or places but there are still not implemented. And to answer your question I think the 95% of quests have a RPG Basis since almost never is lucky thing or random thing , maybe now with spoilers all things sound so easy to make but you have to know that before you only have NPC conversations , books and some hide clues on the map. The "random" quests are a reward for those who explore and try everything to find new things , so yes there are some Non-rpg basis quests but are a low %.
My personal opinion about quest is that they lost all sense since when they implemented spoilers on wikia and even CM's giving clues or even giving full spoiler.
But there is still 1 quest that not many people knows that hold the spirit of investigate , solve puzzles and try to find new secret places only by investigating and reading books without spoiler.
This quests have been started on the first part of the Summer Update 2013.
The quest started without any spoilers or clue , not even appears on teasers or anything , they only started it by implementing some new squares in the ground on hole tibia like this one:
Where you can see a little holes, some are bigger and another are smaller , biggers was found that means 1 and smallers means 0, with this and with the help of a lot of people community found that there was a binary code, on this case the result was.
And if you transform that UTF-8 character set to words you will found the word "Venore" and that words without the all words that you can decrypt you got some clues to follow the quests.
This is an example of quest that still have the spirit of the real old quests without spoilers.
Btw this quest still remains active and last part was found on this summer update giving a new objet called "The Cube".(The use of this object is still unknown)
If you wanna know more about this quest you can found the spoiler here --> Spoiler
I really like the history of quests on Tibia and investigate on things like this , I hope that future quests will be harder and without any spoiler so people will have to do it to the old way.
Hope my answer helps you , if you need more just tell me in the comments below and I will help you !