Three related videos of Brady Haran in Italy interviewing Brother Guy Consolmagno, director of The Vatican Observatory, were released today on his various channels... and rather than submit them individually:
The Pope's Astronomer - Sixty Symbols
The Pope's Telescopes - Deep Sky Videos
The Pope's Space Rocks - Objectivity #221

You've been asked to submit a proposal for a space exploration mission of your own desire, to the New Frontiers spaceflight program. These missions have a cost cap of approximately $700 million to $1 billion, and have famously produced the following spacecraft:

• New Horizons, a flyby probe to Pluto.
• Juno, a polar orbiter of Jupiter.
• OSIRIS-REx, a sample return mission to a rocky asteroid.
• Dragonfly, a drone lander to Saturn's moon Titan.

These are medium-sized missions in both scope, and cost. You can't build the Mars 2020 Rover, or the James Webb Space Telescope. What do you send, and where? Things to consider:

Technology Readiness Level

Administrators are less likely to choose your mission if you choose to integrate risky or untested flight hardware, or novel concepts into the mission design. You're more likely to get selected with more conventional hardware.

Power Source

Your best bet is probably solar panels, maybe something commercial off the shelf like NG's Ultraflex panels? The downside is that these are only effective up to about Jupiter's orbit, and generate power according to the inverse square law. How much do these cost and weigh? How much energy do you generate?

If you go further out into the solar system than that, you'll need a Radioisotope Thermoelectric Generator (RTG). There aren't many of those around, in fact, after Mars 2020 has taken its RTG, there's two left. What makes your mission deserving of an RTG? Is there enough power in the MMRTG to power your mission?

Propulsion

Does your mission need in-flight propulsion? Either for orbit insertion, landing, or maybe a long coast with Ion thrusters like Dawn? If the latter, you can get some pretty good Xenon-powered thrusters, like NEXT, which gives you 236mN of force from 7kW of input power (this rules out an RTG as your power source).

Don't need long-term burn capability? Maybe a COTS bipropellant engine like LEROS is your thing. Watch your weight though, bipropellants aren't efficient! Often more than half the mass of large spacecraft can be dedicated to just propulsion alone.

Instruments

Go crazy. What are you looking to research? Do you need a long range camera, a wide angle camera, something outside of the visible spectrum, a spectrometer, ground-penetrating radar? Do you have a mass-budget in mind?

Launch Vehicle

Every dollar you save on your launch vehicle, you get to add to your mission profile. Your best bet in terms of performance and cost is probably Falcon 9, which retails for $62-90 million, depending on the amount of assurance for success you need. Of course, if you can find a cheaper launch vehicle, feel free to pick it if it fits into your mission weight.

Objectives

What scientific questions do you want to answer? What are you interested in exploring the most?