The item spawner is already creating more interesting set of items which are a lot more unpredictable than were the old level theme based items. There is a resolution problem in finding free places for objects. It's a classic chicken-egg -problem: if you first find places for objects it's going to include unwanted places where some objects could be created. For example monster search includes water tiles where some flying/swimming monsters can be created, but not the land crawlers. Then again, you can't select a monster first and search after that, since there is no guarantee that such place exists (not all levels have water).
The solution could be two or more search patterns combined with specific set of monsters for that place type. I can think of four such scenarios: terrain/cave, forest, beach and water. Maybe the level creator could actually determine those places automatically by searching and counting the size of each area type. That's surely something to try with monsters.