This small, largely neglected, force in 40k is really in an unfortunate spot. With Sisters of Battle on the Horizon in 2019, and Forge World not really doing much for them or 40k in general, they really can't and won't get any love in the future. They are stuck in this perpetual state of meh, and the current rules surrounding them make it even worse.
If you don't know much about them, they have 3 Elites Choices. You can take them in a Vanguard Det of JUST SoS, but gain no command benefits (this includes CP). Combine that with a somewhat mediocre stat line AND the fact they are Index only, and you are left with virtually no reason to play them.
And while it would be great to get prosecutors with the dual pistols and their sweet FW transport, there are really only 3 changes, NONE OF WHICH REQUIRE NEW MODELS, that GW would need to do, and could do OVERNIGHT with a simple update to the Index FAQ
CHANGE 1- Make Prosecutors Troops- Simple change. If Custodes have troops, they should to. They are just Bolter ladies. But critical in order to make them playable (ie, get around the rule of 3) and useful in context of the next 2 changes.
CHANGE 2- Add an HQ Choice- GW should do exactly what they did with Custodes: make it so you can make an HQ out of the box. Essentially make up a Knight Commander Stat-line, just like they did for a Shield Captain, and allow it to pick the wargear in the box (sword, flamer, bolter). Give it some special ability, probably an enhanced Deny the Witch or something of the sort.
CHANGE 3- Add an ELITES Choice Command Squad- They key here is a squad with min squad size of 4. Limit- 1 per knight commander. See what I did there! You get 5 in a box! If you make a Knight Commander, you'd have 4 models left over... PERFECT FOR A COMMAND SQUAD. Probably stick just with Bolters and give them a 2+ BS. Or maybe same stat line, and allow mixed weapons. IDK. Point is to make a 4 woman squad possible.
Simple. Done. All can be done via an FAQ. You're welcome GW. No credit needed. Just please implement these changes and make such a unique fluffy little force practical!