Well, it has lots of range (it's a Charger, I think that's a safe assumption), so perhaps Damage Up would help (lower charge time to the2-shot should help), and Ink Saver Sub so you can get more shots after throwing out a Splash Wall. Ink Recovery Up and Saver Main might work too. Swim Speed Up would be nice as well, for running away when spotted. Special Saver and Special Charge Up could be nice as well, if you like Killer Wails.
Special Duration Up wouldn't be too useful, since it just lets you aim the Killer Wail longer. Run Speed Up... I'm not sure it's good for Chargers.
It might be best for Tower Control; it has Splash Walls to throw on the Tower, and the range to snipe riders or enemies attacking the Tower from afar. Maybe good for Rainmaker as well, since Splash Walls can block Rainmaker shots (I think so) and Killer Wails can break the shield really quickly. Not sure it'll be good in Splat Zones, probably stage-dependent. Might be alright in Turf War.
It's hard to say until we have it, but I think the Bamboozler 14 MK 1 will be better than the Rapid Blaster, at the very least.