One of my favorite things to do as I replay KoA is watch for places where the world clearly has space far beyond what's used in Reckoning. The fact that this game was made with being an MMO in mind is clear and tons of fun to explore.
One of the simplest, most likely to be ignored places where this happens is in the cities. Yes, all of them. Have you ever noticed how many empty or mostly empty rooms there are? Or how many places where a castle has a section of wall that looks just like a door? One of the problems that MMOs have as they grow is that as new mechanics or events or shops are added, more space has to be forced into already-large cities. "Wait, where did that come from? Stormwind wasn't under construction recently, it's even described in the lore as a very old city...." A KoA MMO would have no such problems because there already are so many spaces included in the game for these expansions, whether unused rooms, not-yet-named or not-yet-useful NPCs, or places where it looks like there should be a room but isn't yet. On a related note, the designers did a great job making the world feel like there's a lot more that you aren't seeing by including rooms such as in Ironfast Keep and Kandrian Keep that you can see but can't yet get to.
Another wonderful thing is how the rooms are spacious yet feel entirely natural because no attention is called to it. For any game, adding gnomes as a playable race is easy because weapons and armor just need to be scaled down and they already fit inside buildings. With the exception of Almain doors and one-size-fits-all chairs (which aren't even smaller in Adessa), a giant that would have its head and shoulders through a WoW or Skyrim roof fits quite comfortably in KoA buildings. If When someone worthwhile buys KoA (I refuse to give up hope), I look forward to playing a Jottun!
The Fate mechanic could also be interesting. Since it's implied in the Spring Lorestones that the mortal races already possess the Talisman of Fate and that this could be what created the Weave (as Ethene implies that not even she can change Fate), there's reason for the Fate mechanic to still exist post-Reckoning. The fight with Tirnoch shows that other things can be done with Fate aside from the typical Fateshift, and I would love to see what sorts of things this could be. One idea I've had is, since an MMO naturally has PvP combat, it could be used much like Injustice: Gods Among Us's Clash mechanic .
And then there's the lore. Aside from the implications from the first thing you hear about any town - "Welcome to Gorhart, the last settlement before the Faelands" - various events and books in-game are constantly pointing to a considerably larger world, from various references to the Icebrine Coast to the posh mayor of Whitestone talking about his home port city to even discussing Eamonn journeying east beyond Klurikon, the world is huge. This place has not one, but thirteen World Trees, the Ring of Keozai. (I want to cite the count of thirteen, but I can't remember where I heard it in-game.)
And my favorite indication of a larger Amalur, the most bone-chilling of all signs....
Tirnoch was not the dragon.
Tirnoch was a dragon.