Really go along way towards obviating the need for a space telescope- Laser Guide stars, adaptive optics, segmented otics, Interferometry, etc.

What makes sense for a follow-on space based telescope is to build a mirror array telescope using interferometry- however, I think there is reason to doubt that the technologies are not quite mature enough for such an endeavor, not only in the interferometry but in construction, deployment, maintenence and design.

Of course, you can make up for lack of technological maturity by dumping loooots of money into the research and development stage. Risk of failure is still a factor, though.

What seems to make the most sense (bang for the buck wise) is to push the newly developed technologies as far as they can go on the ground, and as barriers are reached, evaluate if a space-based platform can surpass those barriers in a clearly identified fashion. If things proceed in that fashion, and if a need for a space based solution is identified, then the technologies will be matured for adaptation into the context of space.

I've heard talk of a ground based 100 meter type telescope with outriggers for interferometry, adaptive optics and a guide star being able to resolve planets around stars in a 30-40 light year range. Obviously, RL kows a lot more about this, but 900 million would go a long, long way towards developing such a ground based system.

Yeti