If you have to ask someone else what to upgrade to it means you have not hit any walls yet with your current camera. That means there is still room to explore and learn.
Hold off on the upgrade. Shoot more with your existing camera and whatever lens / lenses that you currently have. Shoot until you find out at least one limitation that frustrate the heck out of you. Then you will find the answer for yourself.
I will tell you my story for you to get a better understanding:
I started out with an APS-C body (much like your D3100). I had only a kit lens to start off with. Like any other newcomer, the GAS disease hit and I started to want to buy a lot of gears. When you are new, you dream of having lenses like the 24-70mm f2.8 and 70-200 f2.8 and you would think that these are the things to make your system complete. Of course, budget is always an issue.
So I bought a 18-250mm lens, thinking that this was the best solution. I also bought a 50mm f1.7 lens second hand as people always say that you will need a 50mm lens right?
After a while, I found that I seldom use more than 100mm of the long range zoom. I was mostly shooting on the 18mm end and I already had the 18mm end on my kit lens. As for the 50mm, I found that while it was good in low light situations, the 50mm on an APS-C sensor is somewhat a little too tight for normal everyday use.
The more that I shot with the 18-250mm lens, the more I realized that I always found that it was never wide enough for me. I was already at 18mm. I can't go any wider but I felt that I need a wider view for my shots. It frustrated the heck out of me.
So now I know that I needed something wider and it came down to lenses like the 17-50mm, 16-35mm or even the 11-16mm. The 17-50mm is just a tiny bit wider so I struck it off my list. The others were either too expensive or too wide for my taste.
Then I came across the full frame body. It almost 'magically' made everything wider. 50mm became very useful and 24mm on a full frame is plenty wide. So I had my answer. What I needed was a full frame body and my existing 50mm lens could serve as my everyday lens. Next I would need a wide angle and I set my sights on a 24mm prime. Later I settled for a 20mm prime. As I still like to have a portrait lens, I set my sights on an 85mm prime (thanks to my experience using the 50mm on a APS-C sensor).
So I upgraded from my most basic entry level body to a full frame body with the 3 above mentioned lenses. I sold off every other lens I had. That was at least 4 years ago.
Today, I am still shooting with these 3 lenses and the same full frame body. I never felt the need to get any other lenses nor body so far.
your story is inspiring and the conclusion is, get a full frame straight and few full frame lenses and you can settle down. lol.