The main problem is not upgrade sucess rate but grind. It is just not friendly for players that grind hard and thier reward is guarantee
Lets start from grind you can grind for 2 hours and you get few red/blue stones but someone else in 1h can get 3 green and 1 purple, this is not normal and fair, someone who forcing himself to grind and trying hard will lose motivation because here's is no results.
A Friend of mine suggested to add new crystals that will have 100% drop rate and let players exchange them for real stones thanks to that every player who grind will have sure reward.See image below
The crystals will have bind, drop rate of them will be 100%, 1 kill = 1 crystal (of course like today from lvl 55 monster you can drop only one stone of u50,u53,55,u58)
Now we need also add NPC where we will be able to exchange our crystals that also will have bind.
250 X u60 = 1red/blue u60
300 X u60 = White u60
600 X u60 = 1 purple/green u60
-Next thing that we need to do is remove stones from tokens.
-Players should be able to collect 999 stones in one slot or even 9999 if is possible.
-Next is upgrade system
1- Give back broken card (when you selected broken card and you didnt use white stone your item is gone)
2- when you hit broken card you getting downgrade -3(50%) -2(40%)
3- purple/green give only +2(60%) and +3(40%)
4- 100k zeni cost for every upgrade hit (This makes a great zeni sink for the game)
5- Every upgrade fail decrease item durability by 25%, broken by 50% (if white stone is used) you can't upgrade without durability (this is for money balance and another zeni sink)
please remember that upgrade system must base on RNG (3 cards) RNG keeping balance between new comers, f2p and p2w players, someone can hit +15 with 3 stones and someone can't do that even with 100.
We just can't totally remove RNG from the game.
Now players will complain about balance because turtle will drop way more stones than fighter, and they can't move stones to fighter here we need go back to Yardat key and make ACCOUNT BIND and not CHARACTER BIND, The Yardat warehouse key opens a bank slot that can be accessible between characters on the same account
we should allow players to share all items between characters on the same account, cc items, dragon wish accessory. etc