A trade union is legally recognised as able to engage in collective bargaining with employers on behalf of its members.
Workers need trade unions to represent their interests individually (in disciplinary, grievance, sickness management and other processes) and collectively (pay and terms and conditions). One important feature of trade unions is that there is a reciprocal obligation - trade unions represent members who pay union dues, they don't represent non union members (though non union members often benefit from collective agreements made by unions with employers). Unions are more effective when they have union density - the more members, higher percentage of a workforce who are union members the more they are able to leverage this to get the best outcome for their members in negotiation.
There have been women's unions in the past by virtue of sex segregation in the job market - you might look at the history of the Women's Protective and Provident League which promoted trade unionism for women and through the impetus of which unions like the Leeds Spinners were formed.
During the second wave some women's liberation organisations did style themselves as unions (The Chicago Women's Liberation Union is perhaps most famous example) but they didn't negotiate with employers in the way that trade unions do. Internationally, many women's political organisations are styled as unions, but also are not workplace representative organisations.
So, I guess the question is - what do you envisage a women's union to be for?
Is it to represent members legally in situations where the trade union might not do a good job? This sounds like something akin to the medical defence union, and could be an insurance based scheme out of which support for individuals could come. You'd need to look carefully at your subscription and your risks/liabilities, and how to get good legal advice, and how you would define the cases you would represent.
Is it to give members independent advice about employment and workplace matters? How will you 'quality control' and tailor the advice you give? Is it a helpline? A website or portal (member only)?
Is it to lobby and campaign for women's rights? In the workplace? Generally? How does it differ from existing women's campaigns?
I do think that if you want to become a representative of women in the workplace for the purpose of collective bargaining then you are on a hiding to nothing - it would be very difficult to achieve the level of union density to compete against existing trade unions. Small trade unions are rapidly disappearing (merging into other unions) because being small is a definite disadvantage for trade unions. Any new women's union would have to be a 'bolt on' (i.e. an additional subscription) on top of the union representation that workers might feel they need.