I'm a beginner DM running Lost Mines of Phandelver for a group of 4 newbies, and they have a party "familiar" who is really just one of those goober characters that the party thought was funny and convinced them to join.

His name is Goblin Jr, appropriately named that because he's about half of the average height of a goblin. He poofed into existence as a joke to be a referee in a wrestling match against Klarg, the bugbear from Chapter 1 of LMOP.

The party had to leave him in Phandalin when going to Cragmaw Castle, and didn't have time to pass through the town again on their way to Wave Echo Cave so he is still there. Sildar and Gundren departed at the beginning of the dungeon (to make my life easier), and the party requested Sildar to convince Goblin Jr to make a journey from Phandalin to Wave Echo Cave and reunite with the party. I thought this would be a great idea for a one-shot, and it would help my players explore other classes/characters since most have only ever played their current one.

I was hoping for some general advice for designing and DMing one-shots, and was hoping we could brainstorm some ideas. Alternatively pointing me in the direction of any resources for this would be helpful too!

I'm also wondering if a certain party member should play Goblin Jr? I feel like they should have their own unique characters, but then I don't really know what to do with him. Him being a DMPC makes the most sense but I like to avoid those if there are better alternatives.

Any ideas?