I have been pretty glued to these types of discussions recently as quite frankly, I am probably the target market that Lexus would be looking at for any and all of the above (RX350-L, revised GX, new introduction TX). This is my first post here after much lurking.
I think the writing on the wall is something will happen with the GX for the 2017/2018 model year. Although, the announcement from Lexus this morning in that LSS+ will still not be standard on the GX by the end of 2017 is quite an interesting statement. I almost read that as there will be minimal modifications, if any, to GX for 2017 year and a redesign / next gen for 2018.
It feels like there is a place in the Lexus line-up for two unibody platform 3-row SUVs. RX350-L to be the Infiniti QX60 / Acura MDX type fighter, but there is definitely a bit of a gap in my mind in regards to something that could go up against the Audi Q7 and the Mercedes GLS for instance. The latter being much of my requirements in my next vehicle. 85% of my driving is in the city, therefore I really have little to no desire to drive a truck based luxury GX getting 15 mpg, when I could get a Prestige trim Q7 with 19 mpg and much more room and tech, or up my budget slightly and get the GLS.
Those are two vehicles that would check mark all of my needs that are out on the market today, not to mention the upcoming "XT7" size Cadillac based off of the Traverse/Enclave platform I believe. The competition in that segment is just continuing to grow and if Lexus does not put something out there, they will become even further behind the current market trends, IMO.
Although, I guess the question always remains if the GX could continue to co-exist with a larger unibody platform SUV of that stature, or if that would be overkill in the lineup. GX would have issues growing in size a bit to better compete with some of the other truck based SUVs at the price point (ie: upper trims of Tahoe/Yukon (Denali)/lower trims of Escalade & QX80) because it would start stepping on toes of LX not only in size, but there is some overlap in the upper trims pricing of competition such as the Escalade that would rival the LX pricing then too. Then again, the 4runner has had quite a bit of success co-existing with the Highlander in the Toyota lineup.
As far as "TX" - It would be nice to have a true, sport based SUV similar to Macan or Cayenne. One with true "F" roots as you would say. I could only hold out hope for that. If you take the actual "TX" to heart, then it could really be that Yukon Denali/lower trim Escalade fighter because there is an extremely large market out there for these types of truck-based luxury SUVs (especially in TX), they just need to be larger than the current GX, but also less expensive than current LX.
I am glad I still have two years left on my current lease as that will give Lexus plenty of time to put something out into the market that would fit what I am going to need upon lease end, however if I was ending today, I would likely have no choice but to leave the Lexus brand and hop skip to an Audi Q7/ Merc GLS at my price point.