If you could change anything of each series to make it better....

[Super Saiyan Overlord1007]

Change anything while keeping the main concept intact (I.E.).

Original - Yugi & Yami Yugi's adventure to uncover Yami Yugi's past
GX - Duel Academy
5Ds - Riding Duels
Zexal - Rehashing the original's concept but with Yuma going from a shit duelist who couldn't duel his way out of the toilet to a topnotch one
Arc-V - I don't know if anyone barring myself has seen it yet.

No cheating like saying it would've been ended right from the get go. Each series is as long as they went but what would you have done to make it better.

 

[Symbiote]

Better character designs. The retarded hair made me not want to watch the anime.

 

[Super Saiyan Overlord1007]

The character designs didn't bother me too much until you got to Zexal. Weird hair at this point is must in this franchise at this point in time.

 

[Papasmurf]

What's hilarious is that when Bakura transferred to Yugi's class in the manga a gym teacher told him he needed to cut his hair to fit school regulations

 

[Super Saiyan Overlord1007]

Well what anime doesn't have ridiculous hair? On a serious note regarding to the topic the only really issue is the characters of some shows and maybe make a few tweaks to the story here and there.

Original Series - The power trio are the only ones who actually do something, everyone else is either a cheerleader or an extra, so I'd just cut some of them because you could exclude them and it wouldn't really be different. Maybe cut out the KCGP as well. I'd say the same for Duelist Kingdom but you got to start somewhere.

GX - Take pointless one shot episodes out and maybe establish some sort of plot from the get go and give the rest of the main cast aside from Jaden some screentime instead of giving that screen time to supporting characters who appear every so often.

5Ds - Same as GX but to a lesser extent. Aside from the first 64 episodes and WRPG onwards it was already pretty effing awesome. But half of the 30 or so episodes inbetween the Dark Signers and WRGP was just pointless filler and nothing to do with the established plot at that point. Half of it that applied to the story it was great the other half could've been spent developing characters who got the shaft like Akiza. In the first half of the series she was awesome and advertise to be apart of the power trio. 2nd half onwards she's just Yusei's dere dere best friend. A lot more could've been done with her character while still keeping her mandatory crush Yusei and his subtle and at times blatantly obvious hints of reciprocation

Zexal - Make Yuma less of an idiot and make him more versatile instead of spamming No.39 Hope all the time. Ace or not Yugi didn't really on the D. Magician all the time and Jaden & Yusei showed more versatility with their Extra Deck monsters and saved their ace as a trump card. Scrap the 2 or 3 members of his entourage who also do nothing worthwhile to the story. Give Kotori something to do other than rehash Anzu and make a more obvious love struck teenager. Kaito & Shark.... Nah they were cool, they can stay the same.

Arc-V -.... I'll comeback to it later.

 

[Papasmurf]

I'd cut the useless virtual world storyline as the only thing it accomplished from a storytelling perspective is explain the Kaiba family's history while making a children's card game even more excessively important than it was in previous arcs. Duelist Kingdom was nearly abysmal early on due to shitty voice acting and extremely funky rules, and could've foreshadowed Dark Yugi's origins better to fit in with the following arc better. But as we said you had to start somewhere.

BC was mostly alright, not too many complaints. Orichalcos was preposterously over the top, although it would've been good as far as anime filler goes had Dartz not come back to life so many times near the end. Millennium World would've been a good way to wrap up had it followed the manga faithfully instead of the poor adaptation it became (seriously, Yami Bakura almost seemed like a better villain when he took Pegasus' eye than in the anime version). That being said, the final duel was cool.