To put in simple words, because even if I put some of the newer mons they would technically be incomplete without their abilities and the physica/special split. Take a look at Gallade for example. Even though it gains a lot of ATK once it evolves from Kirlia, it's special attack is severely drawfed by Gardevoir's. This would be remedied by Psycho Cut, but the problem here is the lack of Physical/Special split. Another great example of a problem caused by the lack Physical/Special split is Darmanitan. He has an extremely high ATK stat(140), but he lacks a lot in the Sp.ATK department(30). This is remedied by the split as he learns Physical moves, but in GenIII, all fire moves are Special, which means that he will do crap damage for all the attacks with STAB. Another problem is space (Only about 411 Pokemon spaces in total.) I know both problems can be solved with a little bit of hacking, but I am quite inexperienced, to put it mildly.
And about Team Celes, they use a mixture of Psychic, Poison, Fighting, Dark, and Ghost-Types. If I can actually extend the amount of Pokemon to be over 411, then I can possibly add a few GenIV and GenV mons (Right now, I know an excellent place to put the Bronzor and Klink lines.)
-When it comes to mapping, I usually try making several different biomes. Take my Weisswald region for example. Even though White Island is one whole island, it's northwestern portion is a desert, its western portion is a valley with a stream running in the middle, and so on. I've actually become quite good in mapping. Another question, have you used the weather effects yet? Rain makes a great environment for regions that would be swampy/damp, sunlight adds an interesting effect on areas that should have tropical climates, such as beaches or small islands. Sandstorm makes deserts seem more like... deserts, hail and snow adds a lot of effect on northern regions, and I often use the mist/fog on haunted areas, to add a more spooky atmosphere(Which is further intensified by Lavender Town's music). If you just mean mapping in general, then try adding detail to the slightest things you do. Sparsely add rocks and flowers to open fields, make small streams run through the map, play around with AMap.