What is your opinion on the expansions?

Excalibur is an unofficial expansion or, simply put, a pirated version. There are many others like it; I remember Lord of the Rings, and something else about dragons. They are playable, but they definitely have glitches. In Excalibur, for example, the arenas with Chaos Demons and Death Dragons glitched, and there was something else...

and what else is worth checking out, because I'm overwhelmed with choices...

No, you're not overwhelmed, unfortunately. If you only consider the official ones, and even with the pirated versions, they don't offer anything new; they just have a new storyline, which sometimes can be captivating.