A concept for CUSTOM custom games
Basic - Set starting gold, levels Gold/exp gain modifiers Map/mode (access to URF, OfA, 1v1/2v2, Hexakill)
Advanced - Ability to access balance files in-client (!) Dominion scoreboard (ie, each kill reduces Nexus HP) Set map AURA (Crystal Scar, Howling Abyss OR CUSTOM AURA to add bonus AP, AD, Armor, MR, ARpen, Mpen, CDR, EXP, Mana/health/energy/fury/whatever regen) Recall and respawn times (I'm sure I'll add more as I think of them)
(!) meaning no mods or 3rd party programs.
The in-client balance file modding would be like Item Sets (!), you create a balance/change file/page and choose that in the CREATE GAME Game Options screen along with other settings.
This would allow players to create their own balances and custom games. Not as advanced as WC3 or anything but it would be a huge step forward I think. It'd be only available for custom games.
(!) You would choose a champion, it would show their abilities, you click on the ability you wish to modify and edit it's cooldown, cost, ratios, base damages per level, shield level, heal level etc.. Add or remove physical, true or magic damage. Add an armor ratio to an ability or something like that.
I thought of something like this and wanted to see if any big posts weren't already going before i posted mine (1st time on forums) and i had a few similar ideas to what you posted and a few others:
Cdr/attack speed/ move speed/ life steal/ (etc) caps
move max CDR from 40% to a certain amount or change the attack speed cap to 5.00
jungle spawn and respawn timers (baron and dragon up at 1:55 for example, or blue buff every 3 minutes instead of 5)
Possible to be customized for each map