-One Flag CTF mode
Like push/murderball? Single flag flagrun? Make it!

-Laser Trip Mines (engi or demo) which also surprises a conc/speeding scout
Too automatic, more player involvement > less.

-Smoke Nade (pyro)
Napalm has plenty of smoke already, and hurts like a bitch while reducing visibility.

-Flashbang (scout?)
I think adding a slight image freeze/doublevision effect to the conc would be pretty sweet.

-EMP Nade(defuse sentries) for spy instead of gas
Spy's already got enough tricks up his sleeve for stealth or overt sentrykilling.

-Prone for the sniper, when he goes prone he's laying down and get a camouflage, unable to move/or slow like cloaking of the spy.
Doesn't fit with the movement centered gameplay. Sniper's already resented and out of place enough in many opinions.

-Destroyable jumppads
I think it's a good idea, if it's done right. Keeps em out of the enemy base.

-Infection by pickup medpacks
Too greifable.

-Team colored frag nades (sometimes i'd have to wait for 3 seconds to see/feel who's nade is still on the floor)
It's a thought, could be pretty easy to change the halo/trail to show teamcolor. Would probably be useful for noobs, but wouldn't affect league/pickups.. I'd support this.

-A motion sensor for the engineer
Elaborate please?

-A Camera for the engineer which can be received by putting up a dispenser
I think camera has been discussed, but is too resource intensive. I'm already dropping fps during firefights, I'd prefer not to lose more to this.

-Caltrop for the scout to get rid of following enemies
Caltrops were replaced by the jumppad, and have been discussed repeatedly. The main reason against keeping them is because scouts already are masters of movement, caltrops all but immobilize heavies/soldiers... creating frustration with little benefit.

-A buildable mortar instead of jumppad for the scout to build, and be operated by the demo for example
Seems too stationary for the gameplay, but maybe a map specific implementation would be cool.

Maybe ill come up with some more.. please tell me what you think of these
I like a few, dislike others, but I really like that you're thinking about these. Keep it up.

-We already have a one flag CTF, it usually uses a soccer ball and some of them are actually pretty fun, but I wouldn't mind seeing more of them.

-Interesting idea, but I suspect it'd be put on a delay so it's not cheap, and it's have to be put to a very low amount of them allowed, the detpack does the job well enough usually though.

-I can see this only being useful for Snipers, I support it actually, as they do lack a secondary grenade.

-I don't like this idea much I apologize, the Conc is already a pain when you get hit, it usually makes it pretty hard for me to fight back, swirling all over and such. xD

-Interesting idea but spies already only need but a moment to sabotage a sentry, I don't think they need any more Sentry breaking things.

-I don't think this would work out well at all, given most snipers need speed to get their kills without being killed in return, a Soldier could easily kill you will you're trying to get up, and Spies would have a field day.

-...Maybe. ('<
-The Medics can already kill you ten ways to Sunday as it is, do they really need to be more powerful?

-That'd be an alright re-skin by a community member, but not as a project for the FF team.

-I think the Sentry beep for Cloaked Spies works well enough, and most people won't be in your base quietly.

-That'd be amusing, but I don't think as an Engy I'd get much use out of it.

-Possibly, but a charged Conc would do the same.

-Err...maybe I guess?

For the engi to put on a wall, when the enemy passes it'll bleep.
when the spy passes it'll be revealed..

Ever heard the 'sonar' on the SG's when a spy is near?
When you place a SG on the right spot you don't need another movement detection :)

well it would be handy for maps like OpenFire when a class gets up the main ramp, and my sg is on the ledge near our spawn, a enemie could get under it without me or my sg detectiing it, so a motion sensor there would be usefull.
Maybe if only to aim your sentry when the motion sensor goes off??

wow now that i like. Not tried to think of any drawbacks yet. but I think that's kinda cool.

:D