Merging Versioned and Versionless Desktop Environment Tags
I think gnome3 (and gnome2, if we have it) could be made synonyms of gnome. Similarly, kde4 (and kde3, if we have it) could be made synonyms of kde. Before doing this, it would be good to make sure we're not losing significant information from questions that use this (and only this) to indicate what version of KDE they're talking about. If it's just a few questions that would lose this information, they could be edited to add it.
Merging Desktop Environment Tags with Ubuntu "Flavor" Tags
As for merging lubuntu and lxde, or xubuntu and xfce, or kde and kubuntu, I'm not as sure this would work out well.
A question tagged lxde, xfce, or kde is a question that is deliberately about a desktop environment. A tag denoting an Ubuntu flavor is much more general and may not capture that a question is specifically and intentionally related to the associated desktop environment.
In contrast, any user of Lubuntu, Xubuntu, or Kubuntu may tag their questions that way for the following reasons:
They want people to answer their question with instructions that can be followed in their Ubuntu flavor. For example:
Kubuntu users cannot generally use
gksudo to run a graphical program as
root, because they probably don't have those commands. Instead, they usually must (and otherwise still generally should) use
kdesudo. This doesn't mean the question itself is about the desktop environment.
Different flavors have different default text editors, web browsers, terminal applications, default office applications, and other applications, with different interfaces, provided by different packages, and runnable via different commands.
Different flavors have different graphical configuration utilities. For example, when a Lubuntu user asks a question about how to configure users, the answer should not just provide information about using the GNOME Control Center ("System Settings"). The answer should (at least) provide information about how to use the tools available in the flavor the OP is using (in this case,
users-admin, i.e., System Tools > Users and Groups").
They are using that derivative and are not sure if it's relevant to their question. This is related to the first point, in that here it's even more important to provide these users with answers that won't necessarily fail for them due to the differences.
They are asking about general features of those derivatives, or whether or not those derivatives will work well on particular hardware and/or for some particular task. A question about how lightweight Xubuntu is, is not really about Xfce. Xfce factors into it, but it's the whole of Xubuntu being asked about. Similarly, questions about artwork, or support (as not all derivatives have LTS releases, for example), are questions about the derivative and not the desktop environment.
If someone asked a question about Kubuntu's support lifecycle and tagged it kde, I would probably remove that tag and replace it with kubuntu. I think that strongly suggests that these tags have a different meaning and should exist separately, unmerged.
On the other hand, there are all sorts of situations where either tag would do. There are probably fewer use cases where a tag like kde conveys information obscured by kubuntu than the other way around. So if these are to be merged, I think the tags denoting the Ubuntu flavor's name (lubuntu, xubuntu, kubuntu) should be the master tags.