Thief as a franchise began in 1998, Dishonored was somewhat inspired by the first two. The newest Thief is absolutely horrible in comparison, from every game design standpoint excluding graphics and optimization. Sandbox seems to be your primary concern, and in this regard Thief is a disaster compared to its predecessors and especially Dishonored which are both far more open. For example:
I must add that the Thief 2 map drawn there is, along with the final mission of Dishonored, the best stealth mission ever created in my eyes. It's so open ended, you really have so many ways to get things done and this applies to every single mission in the first two Thief games and Dishonored. It's a shipping warehouse, and as you can see many business owners store goods there. Much of it is locked down at the start, but you can find controls for the doors and enter every single room and steal from all these different businesses... or not, if you'd rather just meet the minimum requirements and leave ASAP. There are also so many hidden things on every mission in the first two Thief games and in Dishonored, but in the latest Thief game there isn't really anything of the such worth mentioning. The latest one is not only extremely linear in comparison, but also not nearly as detailed.
But the biggest issue with the new Thief is the contextualized mechanics; just about everything is contextualized. In Dishonored and the classic Thief games you can climb practically anywhere, adding to the sandbox nature of it, and giving you more ways to approach/sneak past things. In the latest one, climbing is completely contextual so you can only climb in a few specific areas where you're allowed to.
The same applies to using rope arrows; in the classics, rope arrows are a type of ammunition available for your bow. Firing such arrows into any surface that doesn't deflect it can be extremely useful, since it drops a rope down allowing you to climb up/down (and you can of course collect the arrow if you wanted it back). In the latest Thief game, you can only fire rope arrows at specific parts of the game, scripted events if you will just like climbing. I think many of the arrow types from the classics were removed as well.
If I recall correctly, melee attacks (or at least sneak attacks) are also contextualized like QTEs. It also utterly fails to capture the distinct, creepy atmosphere of the first two, and they tried to make it more story-focused with the most cringe-worthy writing ever.
So... as for sandbox games with upgrades that I do recommend, how about Fallout: New Vegas for starters? If you liked Fallout 3 (which I advise playing again with mods, as a different type of character, if you haven't already) then you should like New Vegas. It's a necessity to play it with these mods in my opinion:
http://www.gnd-tech.com/content/930-Fallout-New-Vegas-Modding-Guide
I would also recommend The Elder Scrolls series (again with mods, they just make them better), which are open world RPGs with highly unrestricted role playing and plenty of upgrading.