So, Synthesis is a neat life crafting skill for Alchemy. It has two types. RNG and Fixed items. RNG is...there's no real fixing that. I don't even want to think about how to even START at fixing that. But fixed items? This needs to be touched up on. There's very little you can make with it, and the few things you can aren't all that useful, or ask for items that you'd rather not put into them for what you're getting out. To top things off, if items were added to this type of crafting, they'd be plagued with terrible item 'grades' anyway if they were gear.
Look at these grades!
https://wiki.mabinogiworld.com/view/Synthesis_Grades They're terrible! The negatives are almost always higher than the positives. None of this is based on rank, and that's fine, but the negatives are god awful. I do like that they're there, but not at the high rate they currently are. (This can also somewhat fix RNG synth a little too indirectly).
Here's the list of items that can be made, some of which aren't even in our version and were implemented later.
https://wiki.mabinogiworld.com/view/Synthesis_Fixed_Recipes_List
This is terrible and small. It's not excusable either. Alchemy has been around for a long time on live, and this skill was NEVER shown any love. It's forever this skill that you use to dupe a few items here and there and nothing more. The one SM based around it isn't even that great due to the heavy RNG with low rates. (I mean, I like it but I can see how awful it feels)
Now, I'm not a game balancer despite how much I throw out ideas left and right. I'm an Ideas guy, and while those are a dime a dozen, it's still worth something to see what sticks in peoples heads. A small few others around at the time of making this agree a bit that Synthesis is pretty...not great. Expanding this list and changing some of the items needed for some of the items currently ending would be a great start!