Great idea.
Even just manually edited local file would help Immensely - at lest for me, I dont mind editing few files to give myself a lot of convenience.
But, well implemented presets can do a lot to make alternative play modes convenient and popular.
Same thing can be used to let people store their favourite map/options settings (so, not limited only to custom modes, but to usual game room hosting as well).
What i mean is having a buttong "save room preset" and "load room preset", where clicking save preset asks for entry of new name (me typing "config1"), locally stores presets of currrent room under something that would not conflict (maybe "niktu_config1"). Maybe would be nice if it stored preset of room that was played in (hence preset creator playername prefix).
While it gives conveninece to players, it also allows of tracing of most popular preset/mods in last 2 weeks or the like.
That in turn would allow game selection to have 4 sections:
-few default and few curated play modes
-top 10 presets in last 2 weeks by time of play or number of games (maybe different colour to differentiate from defaults)
-recent 3 presets of recent games of current player
-local presets saved by player (possibly dozens) and saved from games entered by player (maybe even more)
Since last option count might be lenghty, it might be good candidate for separate option with search as you type dialog.
The general idea behind it all would be to make using players few own favourite mod/presets as quick as using one of default modes, make discovery and propagation of more popular mod/presets fast, and stealing mod/preset of that fun game you just saw eforrtless.
Bonus point is that making new curated mode might be just quick rename/edit of file that is already circulating as popular private preset (yeah, right, after all that dev that woudl go into tracking presets of recent games i have audacity mentioning that, ahahaha)
Well, before I ran too far, I meant to say that manually editing file would be awesome, but button to locally/privately save/restore presets would help people that dont like to dig under hood.
... and everything else was just GoogleFrog giving a finger and Nik taking both hands and a leg, and running away while maniacally laughing, ahahaha :P