The Netziv understands the warning, "U'shmartem es chukosai v'es mishpatai asher ya'aseh osam ha'adam v'chai bahem" (18:5) not as a general exhortation to observe mitzvos, but as a specific command to be an "oseh", to create and innovate in Torah. We ask Hashem daily in Ahavah Rabbah to give us the ability to "lilmod u'lelameid, lishmor, v'la'asos, u'l'kayeim" -- "la'asos" is distinct from the shmiras hamitzvos of "kiyum" and is connected to the process of limud haTorah. Study, limud, must eventually lead to la'asos, creative insight and chiddush.
The Mishna at the end of Kiddushin that tells us "Avraham asah es kol haTorah ad shelo nitna" may be interpreted to mean not that Avraham practically fulfilled every aspect of the Torah, but that he intuited and was mechadesh halacha even before it was given.
Using this insight, the Netziv offers a brilliant answer to a famous question posed by the Rishonim. The gemara (Brachos 17) darshens the paskuk "seichel tov l'chol oseihem" to refer to those who are "osim lishma", because if one engages in Torah shelo lishma it would be better if he/she were never born. Rashi and Tosfos are bothered by this gemara's harsh critique of shelo lishma when Chazal tell us that l'olam ya'asok adam b'Torah u'mitzvos afilu she'lo lishma as that will eventually lead one to the higher level of lishma. Netviv answers that when Chazal tell us that she'lo lishma is acceptable, they are referring to the study of Torah, the accumulation of knowledge and wisdom. The gemara in Brachos, however, refers to those who are "oseihem" -- "osim lishma", meaning those who are engaged in new interpretation and insight, chiddush. Study can be pursued for any aim, but chiddush and change demand pure motive and intention.
This novel interpretation of the Netziv is echoed by many poskim. R' Moshe Feinstein writes (Igros Moshe 4:49) regarding women innovating practices like wearing a talis, etc. in the name of equality that while there is nothing technically wrong with the action involved, it is a chiddush in the sense of departing from previous tradition, and therefore must be undertaken only with the purest intent and motive. Rabbi Arye Frimer quotes the policy of the Chief Rabbi of Britain in his paper on women's minyan: "The most important consideration, however is the motive underlying the request. If this is genuinely put forward by observant students seeking, as you write, "a religiously fulfilling experience," it is one thing... But if the true intention is to challenge the accepted by symbolic reforms, then clearly greater caution is called for. As a protest action, what begins with relatively minor modifications may well end with far more serious violations of accepted practices. . . . " Rav Hershel Shachter has similarly stated, "How much more so when one wants to be mechadeish to reverse an accepted position, we must be sure that the author of the original idea is not formulating his chidush shelo lishma - just to gain popularity or for some other ulterior motive. Although it is permissible, and even encouraged, for one to learn shelo lisham, for one to be mechadeish shelo lishma is not allowed (see pg. 26 in B'Ikvei Hatson)."
Two final observations: firstly, because of this concern for motive I think in recent times we have seen a reactionary pull away from all chiddush, a la the Chasam Sofer's famous dictum "chadash assur min haTorah". Even where innovation is clearly called for and undertaken with the best intentions, there are those reject it in the name of preserving the status quo. This approach abrogates the ideal of "la'asos" entirely.
Secondly, my wife has observed that the criteria of lishma does not seem universally applied. "Amein groups" and "amein parties" are widely accepted even by those who would frown at the thought of a women's "minyan", but there may be little or no difference in the motivation of the attendees. Nearly every week our inbox has an e-mail from someone advertising their efforts on a local e-mail list to try to get 43 women in total baking challah as a segulah for that week's needs for yeshua -- is this not a chiddush (I am not aware of any source for it in general, and in particular wonder why the modern orthodox community in which I live is so adopting of segulos and minhagim from outside sources) that should be communally adopted only with proper motivation? Yet, practices such as these pass unquestioned. A feminist or cynic would perhaps rightfully wonder if our concern for lishma only extends to areas where the male dominated heirarchy is at risk.
In short, each situation demands careful evaluation. We must personally aspire to innovate and be mechadesh for the right reasons, but at the same time must be careful of crushing others creative insights by questioning their sincerity and motive. A very delicate balance indeed.