Concerning not having playable characters as a boss... umm... X5?
Not as a final boss, and Awakened Zero scenario never happened storywise.
IOW, he can be a boss as long as he is still playable afterward.
I really don't like X4's
:l
SHUN! SHUUUUUN!
X5 had better Ride Chaser music! 
Kraken's theme was pretty catchy, but for a Ride chaser segment, I prefer Stingray. Much more pumping. Felt more epic.
...
I miss X4.

Cyclops is frikkin' awesome if only because of the manner in which X8 stepped up general Ride Armor abilities. LIMITLESS GROUND DASH, invulnerability to most projectiles, and ground-stomping damage. To say nothing of theme music. All very, very kickass.
That was pretty awesome. The ability to freeze enemies in place with the Blue cyclops was kickass too, but its brokenness kind of downplayed the use responsibly thing. In X1-7 you had to make sure you used it right or you'd lose it. especially in X5. Though its almost impossible to really take that much damage on it in Mattrexes stage... honestly.
hmm. Also, the Cyclops' air dash was pretty lengthy as well.
As for gameplay, X8 made Axl more different from X but I do feel that Axl could stand to be touched up. Advent's shpiel of copying bosses would be cool if better implemented. Also, he should be using both pistols in his attacks.
combine X8 Axl with Command mission Axl maybe? I do agree that he could stand to be more than a Bass clone with a hover. Also, his Copy ability has not really been that usefull in either X7 or 8... Though i liked how in 7, while you coldnt copy the form of enemies not your size, you could get an ability from them temporarily. X8 could have used that. only in CM has his transform been extremely useful.
This is what happens in X9.
I SAID SO, THEREFORE IT IS CANON.
I really want Dynamo back. not only was he comic relief, but he looks cool.
Frog was downright terrible, though.
Becase you're supposed to use it underwater. Not on Land.