Within limits you can spawn and remove characters on the fly - however that does not overcome the limitations of the drain of AI think time in what could be described as normal gameplay.
i.e. You are still limited to a very small number of characters to view and active at any one time. This is fine until you consider that its not really very realistic or aiding of good gameplay in many situatuions, game types or gameplay scenarios where in large areas suxch as large rooms, building complexes or outdoors such as a city characters are very sparsely apparent and as hard to find as Big Foot. Its just not normal to play games where you "Always" have one or just a couple of enemies per gameplay scenario.
Its just very limiting to game design and creativity and it makes a large game seem very structured and predictable - not at all helpful in creating interesting gameplay which holds the players attention.
The whole complex issue of AI thinking drain, lagg, poly culling and overall gameplay speeds above all else keeps the product at the bottom end of serious game making products - which is a great pity as it has many good points.
Theres a lot of things that can be done to speed up fps and gameplay, however at the end of the day the major amount of effort required will not overcome that limitation of number of characters to view at any one time.
I'm still working on it - but it seems that even in an efficient level the overall low imposed max fps and other engine ineficiencies make the end users job much more difficult than it should be limiting the resultant games unecessarily to poor results.
You can have a lot of characters to view in an otherwise empty level - but thats not very helpful.
This need not be the case - any improvement to the engine in any FPSC update would be most welcome and every single improvement to fps however small will have a beneficial effect pro rata. Every couple of extra fps you can get will give you one extra character to view during gameplay.
TGC could start with removing the fps max of around 33fps.
At the moment you are stuck with what youve got - hopefully TGC will give users something more in future, but theres no indication one way or another that such will be so.
Theres no option at the moment then but to assume that you have what you have and wont get anything more and either just get on with it and do the best you can - or not.