CanI add other grains or seeds (flax, chia, millet) etc.

i would use cooked farro and/or wheatberries. both would add nutty flavor, and a chewy component. for assorted fruity flavors, you could add dried cranberries and candied orange peel.
i put cumin and fennel in my soda bread...
best,
mindy
le bec fin

Go for it! You could swap out some of the oats for another grain (or seed), or just add some additional mix-ins to the dough to sight.