My players are level 18 on our 2-year campaign. I am in the process of writing a story where they will switch souls. After that campaign, I thought of rewarding all of them with a 1-level dip in any other class. However, I don't want them to be kept from reaching level 20, so I want this dip not to give them that drawback.
Are there any large issues with just giving the players the regular features from that 1-level dip, and then progressing with the remaining class up to level 20?
One hitch I have at least found is spell slots. I'm not sure how to build spell slots for the 21st level up (when the characters are 20/1).
Example: Our 18 Ranger will most surely make a 1-level dip in Rogue, for level 1 Sneak Attack. But he still wants to have the level 20 Foe Slayer Ranger feature in the future.
Our 18 Bard can pick up Cleric for proficiency with Heavy Armor using the Life Domain, but she doesn't want to miss out on her Superior Inspiration after 2 more levels.
This being a homebrew kind of question, feel free to ask for more information if I have not been clear on some point.