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This excellent, thorough and well-sourced answer to Has a gravitational microlensing event ever been predicted? If so, has it been observed? mentions several works where hundreds to thousands of events are predicted.

Roughly speaking these are within the next 50 years and require space telescopes due to the sub mas and sub mmag sizes of the astrometric and photometric microlensing effects, respectively.

The answer begins:

In the pre-Gaia era, this was effectively impossible to do, since positions and proper motions weren't precise enough for a large enough sample of stars. With the release of Gaia data, though, it has become an active area of research.

Next generation GAIA?

A simple scaling-up by a factor $f$ would have a possible smaller resolution by $f^{-1}$ so "look" a factor of $f$ farther in parallax measurements and have an $f^2$ larger light collection so would "see" a factor of $f$ farther given the same limiting magnitude (which may or may not be a valid assumption).

This is certainly a challenge for the CCD array, which may have to be a factor $f^2$ larger physically but (presumably, naively) have a factor of $f^4$ more pixels in order to fully exploit the improved diffraction limit, and that may have serious noise side-effects.

Question: Could a next generation GAIA simply be scaled up by a factor of 3x or 5x of the same beautiful system of two rectangular primary mirrors pointing 106.5 apart, imaging on the same focal plane at the same time on a big silicon carbide "optical bench" slowly and steadily rotating in synch with the CCD readout speed? If not, what would "the next GAIA"-like instrument be like?

Further reading:

GAIA optical bench and dual primary mirrors

above: GAIA's Silicon Carbide Optical Bench, with the two objective mirrors of it's twin telescopes pointing 106.5° apart. From Spaceflight 101, image credit: ESA/Astrium.

GAIA's CCD array, from Spaceflight 101 & ESA

GAIA's CCD array, from Spaceflight 101 & Astrium

above x2: GAIA's CCD array, from Spaceflight 101, image credit: ESA and Astrium respectively.

Gaia's Imaging system, including mirrors 4, 5 and 6, prisms, diffraction gratings, and CCD array

above: Gaia's Imaging system, including mirrors 4, 5 and 6, prisms, diffraction gratings, and CCD array, from here, credit: EADS Astrium.

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A scaled-up version of GAIA 2.0 as just a scaled-up version would be a hard sell to ESA. M-Class missions with ESA do not have a history of following that path. Instead while still trying to keep the astrometry tradition alive, it is more conceivable that ESA will accept some sort of game-changer as next mission.

An infrared GAIA could well be this game-changer. To quote Hobbs et al, (2021):

The use of visible-NIR TDI capable detectors is optimal for maximising the science return by reusing the same well established concepts as Gaia. A new NIR astrometry mission will detect at least 5 times as many stars and open up the important NIR regions of the Galaxy. While the spacecraft is scanning outside the Galactic plane it would be possible to go deeper than G = 20.7 improving the science return in the Halo region. The main challenge for this exciting science mission is clearly the development of the new detector technology at a reasonable cost.

So essentially, as large parts of the galaxy are obscured from view to GAIA due to dust extinction (except for e.g. Baade's window), this effect is much weaker in the infrared (as the galactic dust infrared opacity drops off with a power-law towards larger wavelengths, e.g. Draine (2003)). Furthermore, the redder population of stars is partially not at all visible to the current GAIA simply due to the Wien-cut-off in the Planck spectrum, so Infrared GAIA would not only increase numbers, but access a different stellar population, which is a much stronger science case to make.

So current developments inside the GAIA community and white papers go into that direction. However as the above article points out, there are still technological issues to be resolved, i.e. IR long-term stability which is crucial for the photometry.

From your question

[...] slowly and steadily rotating in synch with the CCD readout speed?

this wasn't quite clear to me whether you suggest this as innovation for the next GAIA, as the current one is already doing this.

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    $\begingroup$ Wow, thank you for your thorough answer! A quick note on the "rotating" bit, since I'd used "same" twice in the sentence immediately before the [...] perhaps I just didn't think a third instance was necessary. It was not meant to suggest innovation; sameness save for scale is what that sentence was trying to indicate. $\endgroup$
    – uhoh
    Commented May 30, 2021 at 13:47
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    $\begingroup$ related to innovation: What was the first use of time-delay integration in Astronomy? Are there instances before GAIA? $\endgroup$
    – uhoh
    Commented May 31, 2021 at 1:00

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