Learn how to merge Tilesets together, from this you'll then be able to pick and choose selected bits from one tileset you like and place them together on one you'll then use with RPG Maker XP. It's something that some people know about, but find it hard to do where as others don't know where to start. Don't panic whatever your skill level may be this tutorial will teach you everything you'll need to know.
Before you can do this you'll obviously need to location two or more Tilesets you'd like to combine, as this is a tutorial and not for practical use I've selected the RTP 004-Mountain01.png and 015-ForestTown01.png as seen below: -*-
This just isn't any old choice, I find that the town set would be very flat without elevations, some sort of approach and surroundings. The tiles themselves are of the same style and won't clash.
Basically the best sets that work together are either made by the same person or people that have added onto a style. Hence why alot of people opt for the entire RTP look, though it doesn't look original it flows.
I put together an image to show how it would be obvious that the sets clash, basically the same three items onto a grass background. Here's the worst flow example:
The items are the same but it's the difference in colours, shades / highlights, saturation and from looking at it closer there's a completely different style of pixels going on in that tree. The RTP looks bubbly, bright and soft where as the other set looks serious, sharp and dark.
If you were wanting every one of your sets to flow to the best they can then a complete re-sprite would be necessary or edit the second one with the objects from the first keeping a close eye on how the second ones are sprited.
Guessing that would be a complete pain for most people, which is why it's easier to select a 'set' (As in complete multiple tilesets) from one genre and or series.
It's brilliant to work with, especially with some of the small features it has compared with some of the unessential ones that Photoshop/GIMP has. I should hopefully be getting round to doing a general GG tutorial sometime in the near future.
Yeah it's already within the Downloads section, I've linked to it as 'Related' on each of my GGTutorials.