At the very least, and this is easy to code, so it should probably come first, I'd like to see a way for the user to control via markup what OpenType features are on/off. XeTeX, which doesn't use pango, but relies to some extent on the same fonconfig/freetype stack, has a simple mechanism for doing this. See RawFeature on p.28 of the XeLaTeX package fontspec [http://www.ctan.org/get/macros/xetex/latex/fontspec/fontspec.pdf].

A more advanced treatment would be to implement logical markup on top of OpenType features, like subscripts etc. XeTeX's fontspec is a good source of inspiration on how this can be done, but if you want a more XML-ish approach, you could take a look at how the Windows Presentation Foundation specifies it: http://msdn.microsoft.com/en-us/library/ms745109.aspx

I'm also going to add that many OpenType feature have a "UI default", and it is nice that Pango turns these one, since many of these features are optional, it should also allow the user to turn them off without resorting to antics like changing the language to something that doesn't have the feature.