After a 2022 freeze, we will finally see a new powerunit formula in 2025 or 2026. And looking at al the news an developments in other racing series and the automobile industry, we can be certain that lower cost will be the major target of the new engine. Although is very unlikely internal combustion engines are gone the next decade, the big manufacturers have stated, that they will not spend major amounts of money in development of new engines. The current technology is good enough, even if they need to run bio/synthetic fuel. So to keep current manufacturers or attract new ones, the new powerunit will has to be much cheaper and development limited.
Another certainty is that it will be a hybrid. Electric mobility will become significant this decade and all manufacturers will promote their electric or hybrid technology. F1 has been hybrid since 2011 and future will be probably more electric and not less.
So my speculation for 2025:
1.6 V6 Turbocharged ICE
FIA standard exhaust heat energy recovery generator
1.6 V6 again because they can keep existing gearbox and rear suspension. Turbocharged because it is more efficient and lighter and by using a conventional turbo, smaller manufacturers should be able to develop such an engine. A conventional turbo will not produce current high boost pressure, so to compensate the powerunit should hit 18000rpm. Imagine the sound
FIA standard exhaust recovery unit, because currently the MGU-H contributes for over 4MJ electric energy every lap. It makes the hybrid system much more effective than MGU-K only (1-2MJ). We have seen that already with KERS 2009-2013, where in that time the gain was mostly lost to higher car weight. A spec recovery unit should be a cost-effective solution to keep all the benefits, without starting another battle for recovery gains, like we have seen past 6 years.
A 250Kw MGU-K should keep up with Formula-E and WEC powerlevels.