EASY is the keyword.
I consider myself to be a reasonably good scripter, and I shudder at the complexity of some of these suggestions. We're after things that will encourage MORE people (and people who have less scripting experience) to join in.
If you want such a script for your own game, post in the Script Requests forum. If you get a good number of people joining in on the discussion and talking about best approaches (and coming up with ideas, not just obstacles to be considered), and a few saying they'd like to try it, THAT script idea would be a GREAT candidate for a code-off challenge, as obviously people find it intriguing, challenging, but not too daunting.
This can be somehow remedied if there are some such existing scripts, as even a less experienced scripters(like me) can learn at least something from them.
For example, if the next code off challenge is a stat distribution script, we(not just less experienced ones) can learn from some such existing scripts:
http://forums.rpgmakerweb.com/index.php?/topic/4309-stat-distribution/?hl=stat
http://forums.rpgmakerweb.com/index.php?/topic/28587-stat-points/?hl=stat
http://www.rpgmakervxace.net/topic/15695-d13x-insane-spds/?hl=stat
(Of course Dekita's one's just outright insane lol)
Off topic: I'd doubt if anyone can beat Dekita for this one
In case of a Combined Skill script, I'd suggest studying Victor's Cooperation Skills and Moghunter's ATB System(as it has a cooperation skill feature):
http://victorscripts.wordpress.com/rpg-maker-vx-ace/battle-scripts/animated-battle/cooperation-skills/
http://www.atelier-rgss.com/RGSS/Battle/ACE_BAT02.html
Sure, the former's designed specifically to an animated side view battle system while the latter is a part of an atb system, but we can still learn something from them about how good combined skill scripts can be written.
Those wanting to know how a combined skill script shouldn't be written can study mine(although it's completely stand alone):
http://forums.rpgmakerweb.com/index.php?/topic/22375-doublex-rmvxa-unison-skillsitems/
They'll quickly find that it's just a complete mess in terms of implementations even though it still works XD
As a disclaimer, I'm not saying that suggesting something really hard(like writing a SBS from scratch) is good for the challenge, but maybe sometimes it's also hard for idea givers to realize the difficulty of their suggestions
For suggesting ideas for code-off challenge, I'd say that maybe a script mimicking Warcraft 3's Devour Magic will turn out to be interesting yet not so hard as well. I call this Consume Buff/State for now.
(
http://classic.battle.net/war3/undead/units/destroyer.shtml)
Translating that into RMVXA, maybe a skill can consume all targets' states, buffs or debuffs, and each will give the user some hp, mp or tp and their details can be specified by each state, buff or debuff.
This can turn out to be an interesting trade-off if a skill only consume negative states and debuffs. The users can gain some hp, mp or tp, but the targets will be free from those negative states or debuffs.
This can also be applied reversely. Maybe a skill only consume positive states and buffs, but each consume will indeed reduce the user's hp, mp or tp. That can be another interesting trade-off.
Actually I'm writing such script myself and for me(a nub scripter) it's much easier than "improving" an ATB script(it's driving me mad already), so I hope(subjectively) it's easy enough to be included in the code-off challenge.