From what I surmise, the biggest arguments against stock outer planets are:
1.) most players will never leave the Kerbin system, or will not exhaust the planets already available
2.) Dev resources are better spent on other solar systems,
3.) Exploring the Outer Planets will just be like Jool, not novel enough
4.) Mods serve this purpose fine.
And these are very reasonable points! However, after thinking deeply about the topic for a while, I have to come out in favor of at least one new stock outer planet. Here's why:
1.) "Most new players will not exhaust the Kerbol system" - I understand this point, but this could be levied against interstellar travel as well. If new players are not going to explore a hypothetical GP2, they certainly aren't exploring systems light years away, needing exotic technology to even reach. KSP2 should be catered to both newbies and KSP1 vets- and one consistent thing that the KSP1 community has wanted for *years* is GP2. Besides if the new stock outer planet is a Planet 9 analogue, it could be a way to introduce players to interstellar technology- in IRL, Planet 9 "is close to the transition point where chemical propulsion approaches its ... limits, and alternative advanced propulsion systems (e.g., NTP and laser sails) become more attractive." Thus, it can be a useful "tutorial area" to test out exotic interstellar systems, without the decades of travel time and less intense mission planning.
2.) "Dev resources are better spent on other solar systems"- Other solar systems are indeed exciting, and probably the biggest draw of the game. However, interstellar travel is potentially *several years* away- not only do potentially dozens of planets need to be developed, but interstellar technology and other systems will have to be overhauled, a stock outer planet is comparatively less resources, and can serve as a stopgap to keep returning players interested during stretches of development. Furthermore, a stock outer planet can serve as a proving ground for systems to be used in other solar systems- ring systems, active geology, etc.
3. ) "Exploring the Outer Planets will just be like Jool"- I agree having ~4 outer planets might be a bit much- Neptune and Uranus are probably too alike each other to warrant separate KSP2 analogues. However, having more outer planets would open up new mission plans and strategies we do not have with just one. In IRL, Jupiter is the gateway to the outer solar system, rather then it's end. With a few more outer planets/planetoids, Jool takes up this role as well. From Voyager-style grand tour missions, to Jool 5 + GP2, whole new avenues for challenges is opened up. The moons of a hypothetical GP2 could provide interesting new missions as well. We don't really have a Triton analogue in game, nor a Io analogue. Landing on a world with an extremely thin atmosphere could provide interesting challenges not found elsewhere in the Kerbol system, and an active volcano world like Io or Enceladus could provide interesting mission plans (like flying through volcanic/cryovolcanic plumes), and also, as previously stated, can test systems which will be used in the future.
4.) "Mods serve this purpose fine." Yes, this is true, and probably the reason why we won't get a stock outer planet. Nevertheless, I think locking an outer planet behind a mod is not desirable. I feel like at least one stock outer planet would open up a new avenue for stock mission challenges, a la Jool5, while if it were a mod, would have less impact. Further leaning into the idea that a new outer planet could serve as a stopgap during development, a new planet would provide a flurry of attention which is more tangible then new mechanics or systems, which can only benefit the game in the long-run. And if you don't like the new stock outer planet? Then you could just download a planets mod.
Not getting a new outer planet is not a dealbreaker for KSP2 for me. Nevertheless, I feel like implementing this long-requested feature is not only a positive for the game as a whole, but a worthwhile endeavor for the dev team. Our solar system doesn't end at Jupiter, and even after a decade of KSP, in my eyes the Kerbol system is still frustratingly incomplete.
