What:
Make catching rats & chompies give hunter XP and/or require hunter levels to catch. The easiest change would be to give "token" XP, eg 5xp a rat or chompy, so they would never be efficient training methods but would be associated with hunter.
The more controversial approach would be to also add requirements to those activities & make them semi-relevant training methods for their level.
Also give quests like Big Chompy Bird Hunting, Ratcatchers, & the Evil Dave / Freeing Skrach RFD subquests hunter XP rewards that could be reclaimed by players who have already completed those.
Why:
Hunter was added in 2006, after Big Chompy Bird Hunting (2004) & of course cats (Gertrude's Cat 2003->Ratcatchers 2005). If these had been added in the opposite order, I'd wager catching rats & chompies would both have been hunter activities. "Hunting" is even in the name of Big Chompy Bird Hunting.
With the retroactive adding of big XP drops to grandmaster quests, maybe now is a good time to add retroactive XP for Hunter.
Make hunter feel like a more complete skill that's well integrated into the game