While editing in UDMF, being able to pick from a prefab list of Doom format/Boom format linedef actions, and the arguments being auto-allocated to the equivalent action.
No need to bother with Generic actions as that'd be overkill, but simply being able to select a line action like 36 - W1 Lower to 8 Above Highest Floor, and having the special and arguments set correctly would be lovely, and likely speed up the flow of the mapping process quite a bit for folks like me who think largely in Boom mapping terms.
