Personally I have tons of ideas for both mods and maps, but I lack something almost everybody equally lacks: time
Mapping wise, I want to make a full SP campaign (and yeah, already thought in the maps and all the features), lots of different places to explore, different themes, moods, full story (fully original, but still based on nali, skaarj, Unreal characters and the overall UT theme), etc.
One of the first maps would be like an AS-HiSpeed2, but the illusion of the moving train would be so strong, you would actually think you're seriously traveling a long distance.
As for mods, tons of ideas, many of them I think I will keep for totally new games in newer engines rather than attempting in UT, since at least one of them it's that complex to pull off.
The ones I still want to try in UT is a full war gametype (kinda like Siege, but much more intense and with much more freedom and stuff to build), a vehicles mod and some stuff for maps (like a full weather system, fog, etc, all from uscript though and nothing native, which is mostly to use to myself and my maps, but I would share it anyway ofc).
I also built a map once (in my mapping early days, knew zero about uscript at that time) attempting to mix: AS, MH, CTF and that atmospheric feel of SP with scripted sequences, although it's ugly, I think its still has a nice concept (ever saw a nali actually catching fish and other planting stuff? you see that in this map)... I might post it here if someone wants to take ideas from it or attempt the idea itself in a better way though.
). Have fun and please DON'T fight!
... So evil that it's nearlly inposible to describe... All I can say is it's related to FLOWERZ, RAINBOWZ and the colour PINK 
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Maybe an underground river running under the bases??