Jake
As Marcus has said, it depends what your planning to add on.
End of the day it comes down to bandwidth. If your thinking of adding 4K or multiple cameras in addition to sonars etc then your likely going to exceed your copper bandwidth limit but this will depend on the copper tether length. If this is the case then moving to Fibre is a logical step. The benefit of moving to fibre is not only bandwidth and network speed improvements but the ability to work with significantly longer tether lengths and much smaller tether ODs.
Moving to Fibre is quite the investment if you want to do it properly, you have to consider things like your quality of tether brought (you get what you pay for), layout of additional components, subsea connectors, topside connectors, slips ring / reel setups, termination tooling, the knowledge to work with fibre faults and repairs in the field, just to name the major points. If you have experience working with fibre then you already have a big head start. If not, its a steep learning curve.
In terms of tethers there are virtually all options available for a price, you can get very small tethers, very strong tethers, tethers in huge lengths, multi core tethers, hybrid copper / fibres, the skys the limit … as long as your wanting to pay for it. In terms of budget tethers there is a company in China i could suggest who make them fairly cheap compared to the rest, but I would not rely on the quality. Using a glass core is never a good idea for small ROV tethers, once you get a crack its all over. Opt for polymer cores which can recover from severe kinks if you can afford it.