"Ryuki" would've been better if the Mirror Monsters actually felt like threats, the story was explained more, and they gave the creatures more personality. The BIGGEST change, though, would be to have Ryuki actually improve, instead of staying the weakest the entire series. I love this show, but it has its fair share of issues.
"Kabuto" would've been better if they just had the balls to switch the focus to Gattack, had made the Worms look more distinguishable, made the Worms actual threats, cut out the unbelievably bad filler, and stopped altering the story every five seconds.
"Decade" would've been better if Decade himself wasn't a walking ball of 'the best at everything,' Narutaki actually played a role that was explained (as an alternative Decade would've ruled), Kuuga actually used his Henshin, and the entire idea kept away from Showa Riders and villains.
"Kiva" would've been better without the Emperor mode.