My first run thru the game was quick so I missed many things. On my second run thru the game, I picked up on few interesring things that kinda points out who the cloak figures in the end are.
1. Keldorn mentioned old prophesy and Cyrics worshippers hunt Childs of Bhaal. This may explain why we encounter some of them during the game, like A) when leaving Drow city the first time, B) a small groups in the forest after you leave underdrak. Both group have very similar members and both have Priest of Cyric. C) After encounetr, one of them will say "slay the Child of Bhaal".
2. Harpers also mentioned prophesy. More importantly, one of the cloak figure said "he is becoming too powerful", hence destory the balance?
So I guess they must be either Cyric worshippers or Harpers?
cloak figures in the end
I see you have not heard the rumor that the guys around the table is actually the developers in their morningrobes, sitting in the interplay cafeteria (lights turned low since they all got bloodshot eyes from spending so much time in front of the monitors), and having a disgruntled discussion that a sequel will be too hard to make. Hence "The child of Bhaal is becoming too powerful" means that they have trouble coming up with a challanging game for 20+ characters and "Worry not. The Child of Bhaal is doomed" is sadly a prediction that a sequel will not be in the works anytime soon. 
I guess they think that if they kill him they will release bhaals spirit therefor freeing bhaal to return. It could possibly be a lead into Neverwinter Nights aswell, but that would be sort of tricky to pull off because of some details, such as if you use your char. from bg1 or bg2 you cant do certain things in NWN. Could it possibly be a council of the gods, or demi-gods or deities or some such? Also, the bhaal symbol might just be for effect. At first I thought that they might of been the cowled wizards but that wouldnt work since they are...dead. Like posted earlier, I dont think a baldurs gate 3 would work, levels would be just too high... They are already exagerated from the real D&D games. The only way it could work would be the story line is post-time of troubles or your new character would be the son/daughter of your old one(making it impossible to import your BG2 char). But then again, that wouldnt make much sense either because "the child of bhaal" wouldnt exist in a post-time of troubles and bhaals blood would fade with more generations. Oh well, now I confused myself. Maybe they didn't mean anything, maybe they are just in the game to make us think they mean something so we keep buying black isle games to see if they will explain it. Oh well.