Welcome to 40k and ATT, Greatergoodness.
That is quite the solid collection you've got there, and as Materpillar has pointed out, there are a lot of options available to you with your current model collection. I definitely agree with him regarding the third Ghostkeel and fielding it in the Optimized Stealth Cadre- arguably one of the top five or so Tau formations (if not top three) when it has a full three Ghostkeels. Definitely run them with the Cyclic Ion Raker- it is an excellent gun and is only made better by the OSC. As for their secondary, either the TL Fusion Blaster or Burst Cannon is good- the Fusion Blaster can wreak havoc on vehicles and Monstrous Creatures, while the Burst Cannon will simply add even more firepower to the unit.
The drone net definitely covers markerlight support, but the best way to run your markerlights is with the Mark'O- a commander with two missile pods, target lock, and drone controller, who can fire at a different unit and give the drones the benefit of his BS. The advantage of this is that, when paired with the Drone Net formation, you have Drones with a BS5 providing markerlights with interceptor, which your Riptides/Ghostkeels/other models with Early Warning Override, can then benefit from.
If you want to branch into FW (depending on what's permitted by your local meta) then a Y'vahra is definitely the way to go in a Marine meta. It also synergizes quite well with the Ghostkeels, as it needs to be up close and personal, resulting in an army designed to go and put the hurt on your opposition.
Just as a note, the Riptide Wing is a very strong formation, but it requires markerlight support to do really, really well. It is still formidable without the proper support, but its effectiveness drops off considerably when all the firepower coming from the Riptides is dropped to BS3.
When we forget we are soldiers, we stop fighting.