Here’s some ideas to custom a engagement ring, that may or may not be possible. It’s important that you check with and rely on the jeweler who will be doing the actual work for their expert advice. Often times, the head or crown holding the stone can be changed to something very striking. Many have unique designs and some even have small diamonds on them. If the ring has been gifted/inherited you’ll want to be sure and have it inspected for any possible wear and tear as it is. Patterns/carving/engraving can be added to the outside of many rose gold moissanite engagement rings to give them a totally different look.
Something that would probably be within your budget would be to set some diamond melee into the shoulders of the setting (on either side of the „head” where the large diamond is set). Melee are small brilliant cut diamonds of 10 points or less that are surprisingly inexpensive. They’re readily available and if you set, say, three on either side of the main diamond will add a bit of flair without having to modify the ring too heavily. They should be bead-set flush with the metal of the existing setting. This should be very possible if your existing ring is a standard solitaire.
If you’re set on opal engagement rings, however, they are usually sold as round or oval cabochons since that is how they show their color best. Be sure to go to a reputable jeweler as there is a lot of synthetic opal on the market which is very hard to distinguish from the real thing unless you are trained to look for the differences under a microscope. Beware of words attached to the stone such as „genuine”, as they tend to be red flags that non-„genuine” opal is to be found very close by! In choosing high-quality opal look for a lot of „fire” (flashes of color when you rotate the stone) and translucence (avoid milky-looking stones). Any jeweler worth their salt should bring in at least 3 pairs for you to choose from.
As for ideas for proposals, I guess it depends on how you want it to play – showy or unobtrusive. My brother in law „primed” my sister by telling her that her Christmas present was big and yellow, then slipped the ring into an opened bill (and envelope) which he asked her to „look at” during dinner on Christmas Eve. By contrast, my best friend was proposed to on bended knee by her longtime boyfriend on the top of the Empire State Building during his band’s trip to the United States.