I have had some great mid lengths, a couple of the best ones have come from Wayne Okamoto. His Dyna-Glyde is a great design, looks kinda like it wouldn't work too good but it is an exceptional performer. Wayne is a great guy to deal with,knowledgable , articulate, returns calls and emails promptly. He makes true customs .Check out some of his stuff on his blog http://oakfoils.blogspot.com/DSC03246.jpg
Sometimes, to get the stoke back, just some extra volume is all that is necessary.
The current trend is shorter and wider. For some a Mini-Simmons does the trick. For me, they just go fast but do not turn well. Here is an example of a new model I have been building a lot of lately.
I would say I agree with this comment. As others said more foam is they key to more fun on small/weak days. Mid length will help you get into a wave and pop up like a long board but does not ride the same a a log and lacks the response of a shortboard. Mid length boards are harder to turn in the critical section of bigger waves and don't trim like a true longbaord so to me they are kind of a "step down" board for guys who learned on a log but can't bond with a shortboard. Mid size boards were a big rage in the early 2000's it seemed but you now see most shapers making short thick baords. Also most of these 70's mid size boards guys are talking about are not liek a modern "fun" board like I think you are talking about. Those 70's modles were shortboards for their time and as such ride a lot different than a fun board. I would go short, wide and thick like a mini sim or fish type board because to me "fun boards" are not fun!
That's certainly one way to look at midlengths. All in all, I think it really comes down to how the board is being used in order to label it a certain way. In other words, Laird Hamilton rides his SUP differently than most of our usual SUP specimens.
My best advice for choosing a midlength would be to call up a shaper - or the above mentioned expert. There are so many options to choose from, so you really need to know what to look for based on what your looking for. So if you want training wheels, you'll get training wheels and so on. I know all of this may seem obvious, but you'll be surprised what a shop employee may tell you just to get a sale. Midlengths are more susceptible to this since they're essentially a cross between two mediums.
I've had the best luck with pin tail midlengths. Good luck!