Open Tibia Server Game Development

There is an open Source Code base in C++ of the game called Tibia. I would like to change and add new features to fulfill my vision of an Arena-Style RPG inside the existing platform. This is a 22 Years old-game that is still alive with a medium-sized, but mature community. I do not have the knowledge to understand how many hours of work will need to be done, but i have a clear cut vision of what i want, i also have a background on Programming, but i have never exercised after my education. There will also be a need to link the game server with a website API, and maybe a Voice service.