Personally my two cents as a die-hard Thief fan (originals at least), I’d love to see a remake of this game but it would do you extraordinary good if you brush up on your dev skills first. Also I am not sure if you plan to release this game (free of course), but if you do and you want to get this game on a major platform like Steam or such, you may have to get the non-commercial license or a written permission from the IP holder to get your game on the platform or at least that seemed to be the case for my game which is also on my signature (not sure about consoles if they allow fangames anymore but in the older days I did see a few on PSN that were free to play in my youth).
Also, I don’t know how attached Eidos is to Thief, but you may want to consider the protective nature of the IP holder on the project you work on in order to prevent a nasty surpise DMCA takedown destroying years of hard work. Luckily for my game, the IP itself is badly tarnished and the IP owners don’t even seem to know they have the IP (because of the indirect acquisition when they bought Global Star), and those that do don’t even seem to care for it from my experience speaking with them. Those “shelved IP” or “lost in the vault” scenarios. Problem there is that my game also may not go very far with publicity on its own without some kind of popular platform (unless I have the proper PR skills and money to promote it or create my own platform but that’s another challenge entirely), and considering I plan to have online multiplayer, I find it extremely important.
If the IP brand is still something very popular (e.g. Halo) than it can complicate matters very quickly and you are probably better off avoiding the idea of making such a fangame entirely, even for personal pleasure.
Hope this helps mate. Many love to bring fandom into a virtual reality and I’ve always supported it but its not always possible for some IPs and if Thief is one of those popular brands, it’s better to save yourself the hassle earlier than to be sorry later. Wish I could say the same for me (spent 12 years, already too late to turn back).