oh man, i have so many ideas...
A champion based on mirrors (reflecting skillshots or even something like a windwall that reflects auto attacks too and makes them bounce in another direction)
A yordle that is part of viktor's Glorious evolution.
A corpse-consumer/collector (yes, we have thresh who collects souls so gameplay-wise this is similar, but it could have a very different theme)
Two champions in one. One of them could be able to use Q and the other E then both can use W and R can combine them into one stronger unit for a certain amount of time. So many things you could do with this, thats just a quick idea.
A person who didn't only see the void like malzahar and kassadin, but was inside it and possibly trapped in it.
A champion with a scythe. Seems simple, but once again theres so many things that can be done with this. Already made a concept for this.
An aura controlling mage. Possibly gets its resource from nearby units? Working on this concept actually
A champion that provides more insight into the lore of Soraka/ Morgana and Kayle.
We finally got a Legion champion (kalista) so maybe a swarm champion. A champion that is a swarm of void insects or possibly the last surviving members of an intelligent insect species. Idk seems weird but im just typing whatever comes to mind. The swarm of void insects could fit very well into the theme of league, especially if they were all controlled by one queen or king insect.
Based on a character from a book i read: a short, sadistic, mastermind with a weapon that can cut through dimensions. Sounds odd, but it made sense in the book somehow...
I could keep going but i have to go. I might update this post later if i can find it.