Yes, I'd love to have more flat baseplates available in a variety of colors - green, gray, brown, blue and tan, but others would be nice.
Raised baseplates are a pain both to use and to store.
On road and river plates, the 32x32 baseplates with a non-studded area in the middle printed as a road or river, there have been a variety of stud sizes. The strips of studded areas on the sides have in recent years been either 6x32 or 7x32, sometimes with a strip of 2 studs down the middle of the road. This is a bit of an annoyance when trying to make a city (sidewalks don't line up). Mainly, though, I'm annoyed that there is less build area than when I was a kid. I had some road plates made to go with classic-space sets that had 9x32 strips of studs on the sides. I don't know why they made the roads wider. With 9 stud areas on either side, this still left a 14 stud wide road area, which is room to have two lanes with 6-wide cars. Yes, one stud more or less of width doesn't seem a huge difference, but when you multiply it out, the difference between 9 studs and 6 is 192 studs worth of build space.
Also on road plates, it's great to have a system. Back in 2002, there were four road plates available, and you could buy them all in 2-baseplate accessory packs:
That was the best plan (though these only had a 6-wide build area), because with these four you could mix and match and make any city layout you wanted. The same is true of river plates, though there you really only need straight and curve pieces.
What I'd really love to see, though, is a LEGO brand mat. Some off-brand companies have made these studded rubber mats:
I would love to see these available from LEGO in green and blue. Yes, they have less grip than ABS, but since these are baseplates, you've got gravity working with you, so grip is not the most important. With a big mat it would be easy to set up, for instance, large battle scenes or to quickly assemble big displays, without having to lay out and clip together tons of 32x32's. Large enough mats could also be put on the floor of a children's play area, and then rolled up when play is done. I could see these being very popular with schools, daycare, etc. E.g. my daughter's daycare has an area with these rubbery mat tiles that fit together that have the alphabet, and in another area they have a roll-up rug that has streets printed on them where the kids play with toy cars.