I guess the most changes will be "under the hood" of SE. Chris Jennings wants to move from the model "AH, plus some other special extensions" to a more general framework where AH is an extension like any other game extension. So there will be API changes for me as programmer. I guess for you as users of the LotR plug-in, no dramatic changes will happen. With the transition to SE v 3, I also want to change the distribution of the plug-in (so it can be automatically downloaded through the SE catalogue browser). This will probably be the biggest change for users of my plug-in.
I didn't have the chance to download the AH game extension for SE v 3 (very lame internet here), so I can't say anything definite. But Chris Jennings has been doing an excellent job with SE and I suppose the next version will also be very good and have some improvements. One improvement is already visible for me: The installer exe is not 12, but only 4 MByte big, because the AH extension is not automatically shipped with it (but downloadable through the catalogue). That's nice if you have bad internet access :-)