Yep! I think in terms of separation, a specified value per localiser as a minima would be fantastic.
My experience with speed control is that if coming in faster than 250, it's difficult to control. If you reduce it, it jumps to 250, then continues down as normal. As soon as you give a speed of 250 or less, the option to give higher disappears. Even at FL130, the maximum I can give aircraft is 250 unless they come in faster. Especially since TMAs can reach very high (FL250 or so in some cases), 250 knots is incredibly slow.
The reason that early level offs are an issue is because the lack of automatic arrival clearances means that airspace has to be unrealistically extended to allow for the user to manually clear them. Something such as autoarr = true would work, unless the main airport has multiple arrival runways. If that were the case, possibly the player could be able to set arrival runway for arriving aircraft before they enter the airspace?
I understand your reluctancy to complicate things, but I think giving airport developers maximum freedom can only be a good thing. It has true potential to be a proper radar simulator - of course consider the path you take, but it's an incredible base at the moment that seems to be only getting better! :)