Let’s be honest I’ve been playing this for years and still have no idea what I should be doing, other than selling any Yelena covers.
Well looking at your roster, I would definitely save up the ISO to max out Black Panther and Magneto for starters. You have them at max covers and they are some of the better 3* characters. After maxing those out, start leveling Kamala Khan for when she gets her 13th cover.