I suspect the IM2 comparison has less to do with screen time as it does relevance to the plot. Black Widow was there to monitor Stark, nothing else. She only got involved later in the film when a situation developed right in front of her and she had to act.
I guess my point is that up until she blew her cover, the part could have been played by some no-name actress and it wouldn't have made any real difference to the character or plot. Likewise I think Diana will be just someone who's just there--maybe an interview subject for Lois and a business rival for Lex if they're going with the Diana Prince identity--and she only busts out the lasso in the final act. Let's just hope they handle her better than they did Talia. Yeesh!
As for MoS2 itself...honestly the first one was so drab that I'd rather they screw it up by stuffing in too much colourful stuff than just getting more of the same.