Anytime. From my standpoint, I want players to make the character they envision. To that end, players should work with their DMs to come up with good starting characters. A good rule of thumb is one starting ability and one starting loot for any character, plus the normal pieces of starting equipment, but those can come from any class or setting. Or substitute a milestone ability or milestone loot in plane or a starting ability. Or grab a piece of epic loot. Or a tier milestone. Or a spell. Personally, I think nearly anything is fair game at character creation, within reason. For example, I probably wouldn’t let someone start with the wish ring, or the spell that heals every part member. Those can come later. But otherwise, my rule is to work with your DM to make a cool starting character. Make a witch hunter. Or a pit fighter. Or a kleptomaniac goblin with a bunch of weird little items (children’s blocks, a hairpin lock pick, and a chicken). Or that pompous battle cleric paladin demon hunter no one can stand. Short answer from above, go nuts, within reason, as long as you and the DM agree, and you’re not trying to outshine or outmunchkin your playing partners.