Nikya of the Old Ways seems like a fun card with an interesting drawback, but strong potential if built correctly. I would like to throw some ideas around with everyone to see what kind of deck ideas we have for her!
Nikya is a 5/5 for 5 mana that doubles our land mana production, but restricts us to only casting creatures. This makes us want to be in a predominately creature build. Fortunately, Gruul is the perfect color combination for creatures, and there are surprisingly still a lot of creatures that essentially will do the same thing as spells for us.
Eldrazi also seem good. Since you will have trouble interacting, I think you need to try to win quickly.
All of these are good ideas.
I feel like winning will be some combination of "Going Tall" with things like Eldrazi, as well as "Going Wide" since you will typically have a higher creature number on the board. Using things like Ant Queen followed by a End-Raze Forerunners feels pretty good in a casual meta.