I upgraded from a T2i to 6D earlier this year and am quite happy.
First think about what you want to shoot and the gear you want to end up with down the road (and the budget to support your habit). Then evolve in that direction, particularly when your equipment becomes the limiting factor. Most of the time I'm the limiting factor.
Bodies are evolving pretty quickly, lenses not so much. I decided a long time ago to end up full frame and L glass, but I couldn't start there. I'm an amateur (but by no means a beginner) and still learning. I shoot family, portraits, landscapes, events, astrophotography, macrophotography, movies, etc. mostly for fun, but sometimes I'm an invited/unofficial/unpaid photographer.
Started a long time ago on film (A1/...) and shot 500+ rolls over the years. Moved to digital with the G1, later a G6. Learned to shoot flash with film, never really been a fan of P&S flash so I used a 420 EX with my G series and it made a big difference (power, bounce, and less red-eye).
First digital SLR was a T2i, but I never really used the kit lens or the pop up flash. The little flash really annoyed me when it would pop-up - I'd prefer a full flash or nothing. Picked up a 24-105F4L IS and 17-40F4L as my starting lens kit. They covered my needs for a while. Added 70-200F4L IS, extenders, and 8-15F4L to expand my focal range. Added a 100F2.8L IS Macro for closeup and copy stand work (digitizing old family photos/albums).
Picked up the 6D earlier this year and soon decided I would probably never use the T2i again so I gave it and the 420EX to my son-in-law to take pictures of the grandkid. Added a 430EX II for better ETTL integration and 35F1.4 L for low light. Picked up a 90EX as a master flash and pocket flash (although I haven't found the need yet).
It's taking a while to make the transition to FF (choosing the right lens and camera configs for a given situation) - maybe I have too many choices
. The 6D is a great tool. I'm happy with the autofocus for my purposes and admit I still prefer to manually focus in some cases rather than figure out how to fool the autofocus to do what I want. The low light capabilities are tremendous, it autofocuses well when I can barely see the subject and would not be able to focus manually anyway. The wireless has proven invaluable for a 1000+ frames on the copy stand and a remote.
I liked the flexibility of zooms when I started out and began to explore a variety of shooting situations. I'm still evolving and adding to my DSLR collection, moving in the direction of adding a few fast primes for specific purposes rather than upgrading my zooms.