Hi,
I've just joined--not sure if this is the right place for Sigil feature suggestions--but figured I'd post a couple:
In "preview" mode (rather than "code" mode), I'd love it if you could right-click a paragraph and and have its CSS style class pop up in an editable text field, so that you could change the class right in the popup. The field could even autocomplete, based on the style classes you have already created.
This one is probably more particular to me, but I'd love to have a counting function somewhere that shows how often each style class in the CSS sheet is used--it would help pinpoint outliers that are probably redundant with others, and/or just slightly different in a way I don't need them to be.
Just thoughts--not complaints. I really like Sigil! Of all the things I've been able to use for free, definitely the one that has done the most to make my life easier.