This has actually come up in the mailing list a few times. I think it would be
great to have this not in the way other browsers offer it (i.e. as popups next
to the input field) but as just another minibuffer completion. So there would be
an interactive command, say, 'form_complete' which either (a) shows completions
right above the minibuffer for the current input field, or (if no input field
has focus) (b) lets the user choose an input field through the hinting system
and then does (a).

By the way, it would be cool to have minibuffer completion also for <select>
elements (drop-down lists), independently of form history. I don't like these
lists popping up at different places at different sizes, sometimes covering
parts of other windows (in tiled window mode). (And not expanding them means
scrolling through the whole list one by one). Additionally, minibuffer
completion is certainly more intelligent since drop-down list offer completion
only with respect to the first symbol of the options.