Could I have a best woman instead of a best man at a wedding?

It's definitely a thing! I've seen it multiple times both ways. There was a Best Woman at a Jewish wedding I went to a few years ago, and I was in a bridal party with a Dude of Honor (his choice of title). Nothing wrong with it at all.

{@jimfarris16] it seems an unusual request but not unreasonable . may be best to ask the vicar who is marrying you what the churches stance on this

It's been done. Legally, all you need are two witnesses to sign the marriage certificate. It does not matter what gender they are.

Make sure that your fiance is ok with it. That's all that really matters.

You probably could.. but I wouldn’t

I don't think your bride would like being the second best woman.

Churches probably have their own sets of rules for this kind of thing but civil ceremonies won't mind

Your wife has to seriously trust you i would think. Thats something you would ask her specifically.

Of course. There's nothing wrong with it...

If you haven't banged her I'll allow it

Stop playing with fire.