Lilac is a bit of a GP name for me as I love flower names but probably wouldn't use it as it's one of the more 'unique' ones. :b
I also tend to group it in with other similar sounding names and compare--I love
Lila and
Leila (LAY-lah), which it reminds me of, and it reminds me of
Lyric as well, which I don't like as much. With
Lilac, I wouldn't pair it with a word name at all because it mind end up sounding like a feminine deodorant product. :-/ The only MN options that come to mind are
Lilac Aurelia,
Lilac Ophelia (over the top maybe? lol), and
Lilac Estella.