to customize petitions could be a useful feature in some cases but I don't believe are too many.
In general, a petition is something that someone suggests and to which others join and this cover the mostly part of cases.
Often, in petition software it's possible to comment, may be discuss in some kind of forum, but only join to the original text.
Or, may be say: "This is the base of text, you can modify it and send to these addresses" and this could be easily achieved opening a mailto in user's client with address and text ready to modify.
But this is a "dirty" system that we don't like, right? ;)
Clearly, the more features the better, but, as you said, life made up of priorities :)
I think that could be possible to divide possibilities in two branch: one to only join, another one allow also customize.
The first part could have option to deferred or immediate sending and, in case of deferred, should be useful to send automatically without need to download and manage CSV that it's something that mostly part of user also don't know that exists :)
The second part, well, it's already working.
Validation email it's something that all serious petition requires but in case of custom and immediate sending each user is responsible of himself (instead, when an association or individual collect signatures, also has some responsibility about how collected and who are signatories) so I think that could be omitted (user send a message from his address to recipients and not on behalf of site/association).
In this case, need only to prevent spam with a sort of Captcha.
What do you think about?
Of course, step by step but this could be the way for a complete plugin, IMO.
Thanks a lot for your work ;)