Err... IMHO, the next yellow summon you should aim for is one that complements your roster of ninjas.
Esp now with the Kage treasures out, you'll have new ninjas with different chases out.
See which summons can complement your chases to extend your combo and you'll have your winner.
(eg. if you get Rhino but have no use for it in your lineup then that defeats it's purpose don't you agree?)

I believe you may have mis-understood me there.
I mentioned that his next yellow summon should complement his line-up, which means whichever line-up he desires but it depends heavily on his current roster.
You can change your line-up as often as you'd like but if the summon doesn't really match your chases then it's of not much use.
(eg. I don't use Giant Katsuyu much as I don't have a lot of strong ninjas which chase low float, etc)