Locking the party will mean you will have to do multiple runs to explore all origin stories. Also, if you pick the warlock guy you will have to go without one of the 4 standard roles through the entire game, which is sub-optimal. Gale/Shadowheart/LAE'ZEL/Astarion provide a well balanced party. Replacing one of them with a warlock make it less ideal.
This is one of the things that concerns me. Also, Heraclea has a historical tendency throughout these games to prefer an all female party. She is willing to forego or tolerate a wide variety of ethical perspectives to achieve this makeup. Right now. since I rolled a cleric PC, Shadowheart is redundant, and worse, there are only male NPCs to fulfil the 'mage' and 'rogue' roles in the party. Would very much like to see female characters be able to take on those roles even if they aren't in the first chapter.
