You'll never get a performance 3.5L in the 4Runner. Not enough low end torque, and to get over 300hp, it has to be running on very clean decent octane fuel. That's why the Tacoma engine is a dog - it has to be detuned to deal with the types of work/conditions most Tacoma owners expect. Which means some gas that is suspect, going past the 10k mile oil changes, pulling up long grades where knock can happen unless timing is dialed way back, etc.
Toyota has taken the 3.5L about as far as it can go without either hybrid assist, or flat-out replacement.
My guess is something like a 2.0L turbo, a hybrid 3.5L standard, or something in between... like a 2.7L Turbo.
I can't see the 4Runner doing anything else at this point. If a smaller engine is used, it will surely be combined with a heavy weight-loss -- like 200-300+ lbs, by better use of steel, possibly some aluminum, and plastics.
Btw, interesting note -- Toyota has milked every ounce out of the 4.0L. When I got my 2018, I was looking at the block and heads, and noticed these weird lines in the cast aluminum. Concerned they were cracks, I googled the subject at length, and came to learn that these are just cast marks - that grow more visually obvious over the years as a carmaker re-uses the same casting molds for a very, very long time.
Ie, Toyota has been pushing the naturally aspirated 4.0L molds as fast as they can without retooling, while they reinvest elsewhere.
I thus think there's zero chance they'll update the 4.0L to dual injection or anything cool like that. They'll just dump that tooling when they make the switch.
I'm open to some other surprising powertrain... but can't see it. I guess there could be some huge surprise, like an electric supercharger, or using the 2.5T from the Mazda CX9 (Mazda and Toyota are working together lately)..... but can't really see that happening.
TLDR version: I predict a 3.5L with e-torque assist to fill in the torque gaps. So like 300 hp, and 300 torque with a smallish li-ion battery under the passenger side and 23/24 city/highway mpg, with a 4200 lb curb weight with 4wd.