There are a couple things sorely missing from synergy that need serious attention. Feel free to chime in if I missed anything you feel is a major issue and hopefully we can get the devs' attention.
1. Abilities are locked once chosen. Especially given the lack of explanation on exactly what any of them actually DO, it is extremely irritating that there is no way to change your ability upgrades. A simple "Reset All" option which refunds the appropriate number of upgrade points would likely be the best option, since it should logically require less work to implement a single menu option or item rather than even more menus to reduce specific abilities one level at a time. If there is some worry about "balance issues", make it thru an item purchased with 10,000 cinders that is only available to buy with capped synergy skill or the like..
2. Lack of viable and/or useful recipes at the highest tiers. I'm sure that the devs could come up with something other than honor augments that could reasonably be added even at this point in the game's life. It makes no sense that there are literally zero relevant recipes at the highest point of the craft.
3. Lack of new recipes in general. Recipes like the Fotias helped keep synergy from being entirely useless, so why not continue adding them for other items which have natural cycles/sets? Off the top of my head we could combine: abyssea satchels, WOE capes, and crafting torques (if you exclude fishing). I'm positive there are tons of other options that would make sense for this type of recipe and honestly surprised they aren't already around. Even simple space-saver ideas like combining certain bard instruments would be a welcome addition for instance.
4. No increase in skill cap. How long has the cap been stuck at 80? Especially given the lack of newer high-end recipes, there doesn't seem to be any reason why the cap can't be raised to 100. This craft has become niche at best, and increasing the cap is unlikely to raise any hackles about lost marketshare from more than possibly a very small handful of players.