Wildfire is okay. It's mostly a new set of harder maps and a group of new mercs (which all got added to 1.13 anyway). I think you can play the Wildfire mapset with 1.13 if you're so inclined.
Personally, I recommend Urban Chaos; it always seemed like the most "complete" mod to me. The team managed to create it before the JA2 source code release, basically just hacking things in, which was very impressive. It has a new storyline/modded quests set in an Eastern European-inspired country, a ton of urban environments, subways, banks to rob, mercs, etc. Starts out as a bit of a scavenging game since you don't get any weapons (you arrive in the country via airplane). Good maps, a ton of content and a pretty active 1.13 conversion
here. Downsides would be some weird/broken voice acting and a dumb goony character "Rude Dog" voiced by one of the developers.
I also remember Arulco Revisited being good, it's focused more on the original experience but with new maps, some more towns, and general rebalancing, but it's not up to date with the latest versions of 1.13, so you miss out on about 1.5 years worth of development. Still worth trying, I think.
Bear's Pit forums maintain a definitive
list of mods, there's a number of others. Russians and Germans have made a few mods but I don't know much about them.
Trivia: JA2 modder
the_scorpion used to post on the Codex but I don't think he's active anymore (in JA2 or here). He worked on a mod called Renegade Republic and did some cool stuff modding in new graphics/animations like a new merc bodytype.
