As I have stated in the other thread, I definitely prefer this way - to minimize changes to OG1 and to aim for an OG2.
In my subjective point of view, there are many reasons for this:
1) complexity of existing code base
(Too big, too vaguely document - as far as I can judge something I have never seen.

Penalized by containing some obsolete features (many of them 4x rewriten due some ad-hoc changes), support for multiple file formats, graphic libraries, etc. When switching to OG2, many of this can be cut, codebase can be minimalized, refactored, prepared to withstand few more years of ad-hoc changes that will come.

)
2) complexity of existing features
(Before adding new features, the old should be evaluated and part of them dropped. A bad example: It's always easier to teach AI how to behave on night turns, if you have 20 unit specials that may interfere with it than if you have 50.)
3) the opportunity to fix (or remove) weird stuff
(There are some unit specials, which implements some very sound idea in a very strange way, that (at least under some circumstances) makes no sense. It cannot be fixed in OG because of backward compatibility. At least for a short time at the start of OG2, we would be able to change stuff freely.)
4) leftovers from PG/PG2, which should be handled differently
(Transport system comes to my mind. Like that you cannot update transport for a unit no longer available (for example from minor nation that you have no longer access too), or all the strange details of interaction of PG2 rules and my old idea of units losing transports at river crossings.)
5) ability to focus
(A controversial thing that may be not very popular, but I believe OG2 should focus more narrowely than current OG, not trying to be universal war similator, but say "I'm about WW2" or "I'm about both world wars" - instead of trying to satisfy needs for anything from ancient rome to sci-fi battles. You can always use a game engine for something it was not designed for, you know - but the main focus should be stated, without unnecessary compromises.)
6) ability to set new file formats
(Although almost all of the existing stuff should be portable to OG2, so there will be enough content for it, the file formats should drop the chains of a 22 years old game and move to current times. Years ago I had the idea to double the sprite resolution (which makes even more sense now when the game have zoom option), and I would go even further - the efiles should be not fixed-length data, it should not be needed to manage generals or medals of whole 80+ nation efile in a single file, and similar...)
7) ability to use new/simplier technologies
(OG2 should definitely not care about such obsolete things like WinXP or resolutions like 800*600. If we limit how many things it should support, we can limit how many time it will take to implement and bugfix it.)
So for me personally the question is not "
Is OG2 a good idea?" but "
Is it realistically in our limits to create OG2?" and "
What can be done for it to be possible?" or "
If we will try to do it, how to do it properly so it will be worth the effort?".