The only new planet we are getting is the Earth's Moon, along with it's new tileset: Orokin moon. By the concept art, it looks like it would likely have a dilapidated Orokin tileset with possible archwing integration.
Either way, Warframe is restricted to the Orokin system until a HUGE amount of lore and gameplay gets added. There was only one other solar system mentioned in the game, and it was not a place where any of us will get to any time soon at all. Minimum several years away, if ever.
There is very few infested planets for a reason, as it wouldn't make any sense gameplay wise if there were more. The game doesn't need a ton of more planets, it needs mission variety (which we thankfully are getting in U18).
With the recent overhype about the supposed "Dyson Sphere" discovered in deep space, my thoughts are this: Why not add the Sun itself as a place to run missions? The sun closed in a massive superstructure built by the Orokin and overrun with Sentient/Other Factions trying to loot its technology. Imagine grabbing a Key to the Sun to get in there.
Gawd I want to work with the Warframe team, just so I could try to put in stuff like that :D
The only problem with the Orokin building a dyson sphere around Sol is that the rest of the solar system would turn into ice planets. All of them would simply cease to function. Dyson spheres enclose the entire star and trap the radiant energy within. Cool idea, but so not good for the system at all.
As for places to go. There are still tons available in this solar system.
The rings of the giant planets for one, and the asteroid belt. Some of the chunks of ice in the ring around Saturn are almost as big as Earths moon.
There could be a half dozen different kind of environs on Jupiter. Its a gas giant with enough internal volume to fill it with over 100,000 bodies the size of Earth. Jupiter also has more moons than have been named as locations, with some of the locations on Jupiter actually being moons. There could be locations that are permanently dark, being tidally locked. Some that never get dark for the same reason, and then there are the levels. You would need an entirely different type of ship to get into the atmosphere of Jupiter, than it took to get to the planet. Then on top of that, an entirely different type of structure with elaborate balast systems to just stay in one place. There really isnt much of a surface as we know it on Jupiter, and even if there is one, the atmospheric preasures would make the pressures we can measure on the bottom of our oceans look like dog farts.
That is just Jupiter. Think about Io. An ice planet, under constant change. Whole mountains of ice moving hundreds of meters every single day as it moves around the Jupiter system. Massive caverns of ice that stretch around the entire planetoid, that fill with water, turn to ice, explode and collapse, then do it all over again. Imagine skimming over the surface in your archwing as Jupiter literally fills the sky entirely, and suddenly a wall of razor sharp icicles shoots straight up out of the ground. Everything is sparkling and catching the pitiful light from Sol, and the ice starts to come back down. An entire hemisphere in motion, ice moving up ahead of you, and ice falling behind and around you. The orange and white bands of Jupiter swirling overhead, another moon whipping across the horizon. There! Off in the distance, a blue energy field flickers under the assault of raining ice, shields struggling to protect the ancient orokin structure sitting on the surface. Picture an Orokin derelict with massive daggers of ice shoved all the way through it, with ice flows packed up against the creamy walls. Hazards all around you. Radiation leaks, energy fields out of control and spilling dangerous energies carelessly through the hull.
Sounds like there are hundreds of places yet to go in our system. That said. I would love to see some alien worlds orbiting an alien sun. Tenno can be put to sleep, so there is no reason they cant survive a really long trip. Man I love space.