First of all, people claim it's confirmed Ace is getting a spin-off blablabla. I've been too lazy to google it so it might or might not be true.
Second: WHY ACE?! He is literally one of the least interesting character to make a spin-off of.The only thing we really know about him is that he's Luffy 2.0 but a little more serious at times. What's the fun in that? All his developments included Luffy one way or another, him alone would just be utter boredom.
I believe they wasted that opportunity using Ace. There are far better characters out there. A redemption series on Crocodile? Sabo and the Rev's fight against the Govs? Lucci's mafia-like climb in the CP-system? A series on Croc's and Iva's past? A series on the Supernova's would have potential too. Ace is just a shitty character to develop Luffy some more and I'm actually glad he's dead. What do you think?