1500 witch hunters (fun)

I think you need an inquisitor to have the assassin.

You don't have all that many faith points. Saint Celestine needs quite a lot to keep her going I think.

It might be worthwhile having something in your army with a gun of some kind in case your opponent fields a vehicle. Exorcists are the obvious thing.

I think sisters could work as a horde army. Could you take Celestine, some exorcists and 70-80 sisters? Much rapid fire bolter death. I think that would be cool.

Agree that the melta guns aren't too brilliant, though giving some to the seraphim may work. You can do fun things by using your hit and run move to go forwards a long way. Don't expect them to kill much though; strength 3 just doesn't cut it in my opinion unless you have a seriously large number of attacks.

It could be cool to have a bunch of arcothingamies. 3 units even. The assassin is ok but not actually legal in your set up and you can't get a daemonhunter inquisitor unless you take a lord, as all your elites slots are gone. A unit of repentia might be ok but the arcos are probably better/funnier.

I'd cut down on the seraphim and make them tank hunters with a couple of melta pistols. Swap two of the penitent engines for exorcists and the callidus and repentia for more arcothingies. If points allow, upgrade the storm troopers to another sisters squad and put the priest with one of them.