Keep in mind that just because the item is legendary doesn't mean that they get to keep it. After it has served it's purpose it could be destroyed or if they do get to keep it then it could be severely weakened or damaged, or it could even be something that isn't meant for them. Obviously if they don't get to keep it they would still get something out of the creation process.
This gives you some freedom to go absolutely bonkers with it since it's only going to be used for a little bit.
Some personal ideas of mine
A Giant Mecha to fight a Kaiju.
A dying god needs a new body.
A Mastersword like weapon. A powerful weapon that works for a time and then needs to recharge.
Are the Warforged a thing in your world? They could literally be the father of an entire race.
Are Airships a thing? If no then they could invent that, if yes they can create an upgraded version. This has a benefit of being a boon for the whole party.
My personal favorite idea, assuming they're into it. Let them design their own creation, they are the inventor after all.
I saw somewhere a homebrew that to use a Potion for Self is Bonus Action or Action. When used a Bonus Action you roll as normal,but as an action you heal the full amount.
He is Vigo! You are like the buzzing of flies to him!