I love the idea of more overworld bosses. A lot of the games in the series (MM, TWW if the big octos count, TP, PH, ST, and SS) had overworld bosses, but I feel like it needs to be taken to the next level.
First off, MM had Skull Keeta, and although it was and incredibly easy fight, I still enjoyed it, and seeing the Stalchildren again was nostalgic for me. However, that particular battle had much more potential, meaning they should have made it longer and harder. When you're in a haunted graveyard in the middle of the valley of death, and you see a giant skeleton standing there, would you be expecting such a short and easy fight? I didn't when I first played it. Except, I am going to excuse this one merely because it was the first bit of experimenting with overworld boss fights, so it's not going to be amazing, and it's quite good considering.
TWW had the Big Octos, except I consider these more of a miniboss. One thing I will point out is that I was expecting some epic sea battle in the boat, but I was disappointed. My overall feelings for the game would have increased drastically if there was a Great Sea battle! I consider it just a missed opportunity.
The only overworld boss in TP I can think of is King Bulblin (there might have been more, but I can't remember at the moment). Although repetitive, the battle was slightly different each of the four times you fought him. The first time, you actually have to chase him around the field, until you strip him of his armour, at which point he moves to the bridge. Knocking a big, fat pig off a bridge was probably one of the most satisfying parts of the game for me XD PLUS, you get to do it
twice, with the first time being drastically different from the first. Using arrows to get through his defenses was a cool and creative idea on Nintendo's part! Unfortunately, though, I feel like these two battles were too short. I think it would have been really cool to have some sort of epic horseback battle (similar to what I said above about TWW). Sure you had Ganondorf, but that was hardly a horseback battle. Again, it's a missed opportunity. Oh well, at least they tried.
The last two battles with King Bulblin were much different from the previous two, and are the only parts in TP in which you actually lose a significant amount of health (although his attacks are quite easily avoided...). These battles were, in my opinion, really fun, but not challenging enough. It would have been more fun if he had more armour and faster attacks during his ground battles (outside the Arbiter's Grounds and in Hyrule Castle Courtyard).
PH and ST also had overworld bosses, but they were pretty simple.
SS brought overworld bosses to the next level. First off, we have the Imprisoned. Although too easy, the Imprisoned acted as the best final boss build up in the entire series! I love how Groose assisted you during these three battles, it made them a lot more interesting, and made each battle different from the last! Overall though, the Imprisoned sort of disappointed me...
After the Imprisoned, we fought Levias and Bilocyte. Just the idea of fighting the great spirit that watches over Skyloft is awesome, but we actually have to save him. I love that they decided to include an epic Loftwing fight in the game, it sort of made me forgive Nintendo for the missed opportunities with the previous games. Unfortunately, however, I found the battle with Bilocyte to be too easy and unoriginal. The whole battle was a game of volleyball, which we've already seen way too many times in the series! But, that doesn't mean I hate it. I found it cool how Levias and Bilocyte were part of the same battle. Losing against Bilocyte made you have to defeat Levias again. I'm not complaining, Levias is a fun battle. I also like how we have to defeat Levias' parasite from on top of Levias itself!
All in all, SS was the first game that really wowed me with it's overworld bosses, but I still expect more. I think the next--or at least a future--Zelda game should include more numerous, harder, and longer overworld boss battles than previous games. A little repetitiveness is Okay too, but don't go crazy like with the Imprisoned and King Bulblin...