PPSSPP needs lots of optimization tailored to the platform its on to work properly. Probably the reason it went to Switch is the Android port is very, very optimized and that would help a lot in adapting it to that console as they are so similar. Not to mention its still current.
Would prefer to see it on Wii U but it would take one or two really talented devs with passion for keeping the console alive, not so much bounties and whatnot because Wii U is so incredibly niche now. Even moreso than the Vita. Most scenes have always a prime mover and shaker (like Lantus on OGXbox) that spurs competition among others coders and inspires motion as it builds up from that.
The lack of action in 2014 when the SDK leaked told you all you really needed to know about Wii U, the latest CFW drive might spur some interest but its sitting in that twilight zone until its hipster value kicks in then we might see some ironic porting action along with waxed mustaches, fixies and expensive coffee when the hardware price has shot up to around 500 bucks, so about three years I'd say.
All of that will be pointless anyway as the new handheld PC market is going to change the game. Speaking of...