I think the Party mechanic is really interesting in fun. I'm excited to be seeing decks for them.
Since the only creatures you have that can be in your party are
Tajuru Paragon, and because you'll need all four out to have a full party, any removal will be able to shut your strategy down. Removal is everywhere, with
Fatal Push being extremely popular, and sideboard options like
Legion's End can come in and hurt the deck greatly. If you want to utilize
Coveted Prize, I think you'll have to add in a wider selection of creatures, with the paragon being a good flex spot. I'm going to be going off that you'll want the main strategy.
You have a number of ramp cards, like
Cultivate,
Roiling Regrowth, and
Broken Bond which I don't think particularly help your main goal. They work with Nissa and
Scute Swarm, but it doesn't necessarily help your main goal. Casting
Cultivate on turn three probably isn't what you'll want to be doing. A fine strategy on its own (and could be quite good if focused on), but I think removing the ramp package and replacing it with more Party centric cards would be best.
That leaves what to put in. Ideally, there should be some early turn plays, and cards that are solid on their own.
Brazen Borrower is an excellent card. Acting as a bounce spell and as an aggressive creature that is a Rogue would be a nice addition.
Archpriest of Iona can end up hitting for quite a bit of damage if it sticks around, and is a solid turn one play.
Linvala, Shield of Sea Gate is a solid card that can protect your board if needed, but can also lock an opposing creature out from anything should you have a full party.
Kargan Intimidator has some aggressive stats and can push damage through quite easily.
Relic Robber, should it get in, can bring in some long term damage should the game last.
Admittedly, I don't know what cards will best for Party yet, and I haven't considered color restrictions. But I think that might be a good place to start.