Regarding "user-defined layout objects" - I'd say the current layouts system is fine,it just lacks few objects to make layouts better until a proper track editor gets in the actual working plans. We're been tricking around with speed bumps and various versions of concrete objects to create some proper curbing on turn insides,but nothing really works perfectly as both speedbumps and slab walls have the trait to cause damage to suspension (sometimes quite randomly) and concrete ramps as curbing are useful just on turns with rather high diameter and useless in for example in Westhill's roads where elevation is a factor.
I personally would like to see some extra objects in the same old layout editor before you start the journey of track editor creation/adaptation for mass usage. 1 meter short speed bumps and 2nd set of same speedbumps half as high would be a relativelly "simple" addition for starters,maybe the lower ones might be bendable. That alone would be a great addition to force drivers stay within designated turn and layout creators would not need to go full hardcore with walls. And maybe some trackside big objects like tribunes,trees and similar stuff could be useful as scenery,but that's least important stuff I guess.
Other suggestion regarding layouts - addition of dedicated spawn slots for objects,so we don't need an extra user to be a ball in a car football game. I hope I could throw an idea here.
And I'm risking to sound like a broken vinyl - with addition of the huge layout square environment,possibilities have grown,so an object limit rise would be very very welcome,might be even limited to layout square area which has almost no other objects around anyway. I would understand a limitation of moving objects like bales/tyres for sake of weaker PCs though.