While it would take a lot more to redo TNG, I think it is the perfect candidate to get the upgrade. It would still be much cheaper to update TNG from scratch than shoot a whole new series.
In 5 years, when HDTV becomes the standard instead of the exception, I'd look for Paramount to make the upgrade. (This assumes ST:11 will be successful and a continued interest in Trek.) From what ratings we've seen, TOS-R has achieved about the same ratings as ENT at a fraction of the cost to the studio. That kind of profit gets studio heads thinking. A ground-up, 16:9, full HDTV presentation of TNG could make CBS some serious money.