Why, if it was mine to decide, I'd probably meld arena into 1x1 and free/advanced/casual, too. (1x1 is a bit of a weird creature just because it won't typically ever recruit after started; the level-division would only really matter with IntChecks...) NRP is just one subtype of RP which I personally have been participating quite frequently (as opposed to approximately one proper arena RP in the past eight years), which is also why it also came up first for me.