So, that brings me to this conclusion: closely-related planes of same family should be clubbed under same tag, to avoid any confusion. But, I have some doubts regarding the Airbus neo. Airbus has already rolled out the A320neo, A330neo and plans to roll out neo for most of its current fleet. Should we have an airbus-neo tag for all questions regarding the neo group?
I'm not an airliner geek and I don't follow things like this, so people who are may see it differently but my opinion is that the tag might be useful for questions about the NEO program itself, but not for questions about specific aircraft models.
As you said, we've already concluded that "closely related" aircraft - usually with the same model number or designation - should all use the same tag. After reading a few random articles about Airbus neo, it just looks like a marketing term for a collection of incremental improvements. It doesn't seem to make the aircraft models fundamentally different in any way.
There's also the issue of avoiding confusion. For someone like me who doesn't know much about airliners, they're all A320s and if I ask a question then how should I choose between airbus-a320 and airbus-a320-neo?
So the most practical use I can see for an airbus-neo tag would be for questions about the NEO program or concept as a whole, not about individual aircraft models. That actually means that it would indeed be appropriate for your question because it seems to be about the overall benefits of NEO, regardless of any particular aircraft model. You did mention the A320 specifically in your question, but if your comments about Airbus offering NEO upgrades for other models like the A330 is correct, then the A320 is just an example.
Personally, I think a re-engine tag (or other similar, generic name) would make more sense than one specifically for the Airbus 'neo' models. As far as I know, 'neo' is just Airbus marketing speak for a re-engined aircraft, which is a concept hardly unique to Airbus. Almost all commercial airplanes that remain in production for a significant duration have periodically updated, re-engined models, especially airliners. The B737 has been through several iterations of this and is currently going through another, for example.
The one and only question currently using this tag could have been asked just as well for all re-engined aircraft, as I expect almost any future question that is not specific to a particular airframe could be. In the case where the question is specific to a particular airframe, I'd recommend using the airframe's tag and the re-engine (or whatever we call it) tag. For example, airbus-a320 re-engine.