There is no way to connect to the rivers of the world, though that would be neat. IIRC, rivers are stamps and do not integrate with Tiles.
Quests cannot be assigned to Tiles.
Tiles are 150x150 blocks in size. They are built in the Prefab Editor like POIs and you place "POI Markers" (similar to Parts) to denote where POIs are to land. The file names start with "rwg_tile_" and then the name of the district to which the Tile belongs, then the shape of the roads on the Tile, and finally any unique identifier (if needed). In short:
rwg_tile_commercial_cap_svarii_01
rwg_tile_commercial_corner_svarii_01
rwg_tile_commercial_intersection_svarii_01
rwg_tile_commercial_straight_svarii_01
rwg_tile_commercial_t_svarii_01
There's a special district related to the entry points to settlements and places in the road system between settlements.
rwg_tile_gateway_intersection_svarii_01
rwg_tile_gateway_straight_svarii_01
rwg_tile_gateway_t_svarii_01
The best way to start a custom Tile is to just copy a TFP Tile's files and then clear off its Parts, Markers, and blocks, leaving the road and sidewalks.
Each District has design considerations that specify street widths, the placement of sidewalks, and sometimes the placement of alleys. There is a document by TFP that covers this. I'm trying to recall where you might find it. IIRC it is pinned in a CompoPack discussion. It might be pinned here in this forum. I'm certain I have a copy tucked away somewhere.
TFP does not have POIs for all sizes for all Districts. For example, they don't have 25x25 (XS) POIs in the Rural District. Adding a size they don't support means you need a strategy for how often the Tile will appear and how many alternative POIs you make available for that size.
POI Markers on Tiles can have Tags. If so, the POIs have to have a matching tag to be eligible for placement. Look at the CountryTown district, for example. Also the Gateway district.
Driveway Parts are defined to match those specifications and generally are sized to wipe out street lights and sidewalks on the Tile to make way for the driveway. In the Residential district there are "driveways" for the alleys in the back which create a gap in fences.
On Gateway Tiles, there are some Parts related to the transition between roads or settlements.
Treasure quests ... if you mean the maps that drop as loot ... are not pre-placed. You can make a custom note but the location will be randomized. Similarly, the buried treasure quests from a Trader have randomized locations.
That said, with a little modlet you can have the chests (blocks) used in buried treasure quests and you can place them. If you wanted to bury treasure on a Tile you could, though players would just have to get lucky to find them. (I've done that with a Decoration, oddly enough.)