A good way to add more abilities is to give players a few extra ACTIVE ability points at different stages as they level up. If necessary for balance, any active abilities unlocked in this manner could have a penalty stamina/mana cost. But tbh allowing people to slot additional abilities which they can get with normal ability points would satisfy a good percentage of people wishing for more than 4 abilities, even though it may cause them to screw up some builds by skimping on passives.
I'll play devil's advocate with myself because I prefer not to read too much dumb replies.
Q: BUT TOO OVERPOWRED
A: Limited by stamina and mana. The classes that can regenerate or recoup stamina or mana very fast (death siphon, AW restorative veil) ought to be nerfed anyways. There are many games where players have access to a dozen or dozens of abilities, but these abilities are simply not used most of the time. In DA3MP, you basically wait for four abilities to come off cooldown, hit them, use basic attack, rotate, yawn, no thinking.
Q: BUT YOU WONT BE ABLE TO USE THEM ALL AAAAAAAAAAAAAAA
A: Yes, that's what keeps it from being overpowered.
Q: BUT FOWR IS ENUOGH
A: You can still just press four buttons most of the time. Once in a while you will see a situation where you want to press other buttons. Expand your mind. Ever wished you had combat roll to get out of fear lock from demon commander? Or dispel to save someone on fire? See a mob vulnerable to ice and wish you had an ice damage attack slotted?
Q: IS RUIN BUILDS! NO REPLAYABILITY! COOKIES EVERYWHERE!
A: There is a limited number of ability points a person can assign right now (19), and only a few more with the suggestion I gave. People would still have to pick what actives and passives they want, and if they get too many actives they can cripple themselves. By giving players the option to use more active abilities, you would see more variety. Some people will choose to use 4 abilities, some 5, some 6, and some not too bright people may try to use 10. This will also make it more meaningful if existing classes were to be given access to more abilities in the future.