I think each character could get a nice 5-20 minute OVA (whichever works better for riot) where you just portray a scene. But not like the xayah/rakan release video. more character based than action based. With voice acting! Maybe lulu leaving the forest to find bandle city totally changed. the ascension of azir, xerath, nasus and renekton. Jhin monologuing about how artistic his next assassination is.
It doesnt have to be the high quality CGI youve been doing, more traditional animation styles like what you see coming out of japan or cartoon network would be fine.