Any suggestions/requests for features for a new package that allows disabling ligatures for (pre)selected words?

There is the perl program rmligs which does this job for German (German being a language where this is needed quite often). Unfortunately, the word list itself doesn't seem to be available. I also don't know how good the list is; it probably follows pre-reform orthographic rules. Maybe you can reach its author. There is a project for improved German hyphenation which has quite extensive word lists. Maybe they can be of help as well.

As for the question whether it's smart to rely on LuaLaTeX I say: go ahead. One problem of the TeX world is exactly how slowly people adapt to new tools. But if no one starts, no one will move. The more attractive tools exist for modern engines, the more people will actually start to use them.

EDIT: And since I'm a huge fan of Stempel Garamond and Frutiger, these fonts should definitely be considered. ;) Seriously though: Couldn't the package just remove wrong ligatures for all fonts and only care about proper kerning for a selected few?

Since you are writing a luatex module, store your data as a lua table so that it may be reused by other formats or programs. For TeX modules, data is often stored as TeX macros, e.g.

\suppressligature{selfish}{self{}ish}

etc. This makes it hard to use the same data in other formats/programs and leads to a lot of duplication of effort. A better solution would be to store the data is a separate lua file as

thirddata = thirdata or {}
thirddata.ligatures = thirddata.ligatures or {}

local suppress = {
   ["selfish"] = {"self", "ish"},
   ....
}

thirddata.ligatures.suppress = suppress

etc. It is relatively easy to read the lua table and generate the appropriate TeX macros, while parsing TeX macros is always a pain.

luatex: As there doesn't seem to be a solution to this problem without luatex, you might as well be hopeful that your package will recruit new luatex users. What's the microtype situation for luatex, does the current release work for it? If not, I'd stick with pdflatex.

fonts: You could take the font poll as inspiration, so Palatino and Linux Libertine would be other fonts to consider.

names: How about smartliga / smartligs or cleverliga / cleverligs?