I don't see any reason why you couldn't make a tactical RPG in Fusion using only default extensions
One advice though, regarding what has already been mentioned about project size for a first game, i don't think a TRPG is impossible with your background. It's more a matter of scope in my opinion
Aim small at first, get the basic gameplay down including graphics and sounds. Sort of in line with the Please login to see this link. approach. At that point, get some testers so you can get some feedback on what does and doesn't work and adjust accordingly
If you get enough positive feedback and you're happy with what you've got, you can now expand your scope and implement things like a detailed world, story, talent trees or what ever else you'd like to have in your game
If you get stuck and start to lose motivation, some studies actually suggest that jumping between a couple of projects can improve creativity. At any rate, it's useful to leave your project for a day or two so you can look at it with fresh eyes. It's very easy to overlook the simplest problems when you've been buried in a project for a while