You've got a good base, but you need to start focusing on characters to get them to 6 stars and level 60. I'd start by grinding Winter Soldier bios in Story mode and Sharon Rodgers for your bio selectors. It has been a while since I've seen them at T1, but they will be your stars once you get them to T2.
You can slow grind the Thor bios once you unlock story 8-9.
Once Sharon is done, work on Iron Hammer.
Cap and Iron Man need their IW uniforms to make them truly valuable, but you can get their bios pretty easy from story mode.
Also, start working on a Special Mission Character. Run it every day at the highest level you can do - I just let it run on auto. I recommend Sandman or Crystal.
It would be better if you mention how many crystals you have. From the above list you should get IW Cap > IW Iron Man > IW Hulk. IW Hulk is better than Cap at T2 but since you already have Cap at T2 you'll be able to get immediate use out of him.
Sharon's getting a new uni anyway so I think you should wait for that, it'll most probably be a meta uni.