Alts and replacements: A simple system.

One of the things bothering me about later editions of D&D is they seem to expect you not to kill PCs... or players to get bored and replace them.

A good DM will come up with their own system for inserting a new character, but the lack of guidelines and built-in rules means its completely roll your own.

Hackmaster and Adventurer, Conquerer King have a similar system that I'm fond of - in Hackmaster, you devote a % of your experience post-adventure to your protege, in ACKS, a percentage of the loot that you outright BLOW PARTYING is credit towards your replacement.

Nice!  But modern players will hate the former, since it seems no matter how much dead weight a player or character is, everyone is supposed to level up at the same time, and the latter works decent in ACKS - it has one of the best rules-sets for an in game economy I've ever seen - but is also 'uneven'.

I've gone for simple.  From session 0 onward, I've emphasized the guilds aspect of the setting - the King has licensed the adventurer guilds as his agents in the Lost Lands, and PCs are all members of the same guild; as one falls or retires, and guildmate steps up.

(Or even better for alt-oholics, your roster can shift from adventure to adventure)

I've opted for dead simple.  New characters come in at the bottom of the prior 'tier' from the adventurers league:

So now that the party has cracked level 11, they can roll in with level 5 alts all day.  Meanwhile they all have a 'main' alt that they adventure with occasionally leveling up on the side.

It didn't come up, but I probably would have been fine with bringing in level 3 alts when they were between 5-10 - you'd be trying to recruit someone that's not totally green.