Let me preface this by saying that I understand mappers will make the types of maps that interest them the most. A mapper that plays CTF will make a CTF map just like a mapper that plays payload will play payload.
However, I'm sure there are some mappers out there that are trying to figure out what type of map they should do next. Let me be of assistance.
I'm basing this completely on the availability of good custom maps in each map category as well as popular sentiment that I've picked up on from the community map vote stuff.
Additionally I'm leaving out odd gameplay types since those tend to either be confusing or not that popular. This includes maps that mix CTF and CP, CP and payload, etc. I'm not saying they're not fun and not well done, but they tend to be less player friendly than the maps that stick to simple game rules.
So, in order of highest demand to lowest, with an example and explanation for each.
1. Branching CP (Gravel Pit)
The only top-notch map of this type that I'm aware of is cp_junction. People love this game type, it's a smaller map type, and it shouldn't be too hard to balance (just focus balancing A/B and RED/BLU in each area), so I'm surprised more of these haven't been made.
2. 3 stage A/D (Dustbowl)
I wish cp_corporation got more attention on the 2nd and 3rd stages since it had a lot of potential. Otherwise we have cp_boulder which people absolutely love (I think it needs more work on stage 3). Without boulder this would be #1 on the list, since I don't think it has hit its popularity peak yet. I think cp_broma fits into this category as well, although I've not played it.
3. TC
This would be higher on the list if TC were more popular. Right now we have meridian and then the "others", which aren't quite finished IIRC. I'd love to see someone try tackling a TC map and figuring out a way to prevent turtling in each area, while still allowing some defense.
4. Single stage payload
I think the payload contest is handling this demand sufficiently.
5. 3 stage payload
pl_hoodoo and pl_cave are the only finished ones. I think pl_haywood shows promise as do a couple others, but overall there aren't that many of them. However, gold rush, hoodoo, and cave seem to be handling the demand for this type, plus the fact that badwater's style seems to be more popular with players and mappers.
6. Linear 3CP
cp_oilfield shows just how successful this game type can be. These maps are fun for smaller games. They're too similar to 5CP maps to get too much damage, though.
7. CTF
There are a TON of good and bad CTF maps out there. I think we could use another official CTF map (aerospace *cough*), but otherwise there's very little demand for these.
8. Linear 5CP
The fact that there are 4 official 5CP maps plus a ton of excellent customs in this style (redstone, switchback, labor, freight) means there's very little need for more of them. The fact that freight and switchback have received so little attention despite being great maps is proof that this category is saturated. In fact, if someone is planning a 5CP map I'd suggest changing it to a 3CP map or accept the fact that your map will rarely get played. Unfortunate but true.
Please note that this list isn't complete and relies on my opinion. I'm not insulting any maps that I didn't refer to in a particular category, but moreso trying to make generalizations.
Hope this helps sway a mapper toward a game type at the top of the list.
EDIT: I forgot arena, which would fall under 6 since there's a decent amount of really good custom arena maps already.
However, I'm sure there are some mappers out there that are trying to figure out what type of map they should do next. Let me be of assistance.
Additionally I'm leaving out odd gameplay types since those tend to either be confusing or not that popular. This includes maps that mix CTF and CP, CP and payload, etc. I'm not saying they're not fun and not well done, but they tend to be less player friendly than the maps that stick to simple game rules.
So, in order of highest demand to lowest, with an example and explanation for each.
1. Branching CP (Gravel Pit)
The only top-notch map of this type that I'm aware of is cp_junction. People love this game type, it's a smaller map type, and it shouldn't be too hard to balance (just focus balancing A/B and RED/BLU in each area), so I'm surprised more of these haven't been made.
2. 3 stage A/D (Dustbowl)
I wish cp_corporation got more attention on the 2nd and 3rd stages since it had a lot of potential. Otherwise we have cp_boulder which people absolutely love (I think it needs more work on stage 3). Without boulder this would be #1 on the list, since I don't think it has hit its popularity peak yet. I think cp_broma fits into this category as well, although I've not played it.
3. TC
This would be higher on the list if TC were more popular. Right now we have meridian and then the "others", which aren't quite finished IIRC. I'd love to see someone try tackling a TC map and figuring out a way to prevent turtling in each area, while still allowing some defense.
4. Single stage payload
I think the payload contest is handling this demand sufficiently.
5. 3 stage payload
pl_hoodoo and pl_cave are the only finished ones. I think pl_haywood shows promise as do a couple others, but overall there aren't that many of them. However, gold rush, hoodoo, and cave seem to be handling the demand for this type, plus the fact that badwater's style seems to be more popular with players and mappers.
6. Linear 3CP
cp_oilfield shows just how successful this game type can be. These maps are fun for smaller games. They're too similar to 5CP maps to get too much damage, though.
7. CTF
There are a TON of good and bad CTF maps out there. I think we could use another official CTF map (aerospace *cough*), but otherwise there's very little demand for these.
8. Linear 5CP
The fact that there are 4 official 5CP maps plus a ton of excellent customs in this style (redstone, switchback, labor, freight) means there's very little need for more of them. The fact that freight and switchback have received so little attention despite being great maps is proof that this category is saturated. In fact, if someone is planning a 5CP map I'd suggest changing it to a 3CP map or accept the fact that your map will rarely get played. Unfortunate but true.
Please note that this list isn't complete and relies on my opinion. I'm not insulting any maps that I didn't refer to in a particular category, but moreso trying to make generalizations.
Hope this helps sway a mapper toward a game type at the top of the list.
EDIT: I forgot arena, which would fall under 6 since there's a decent amount of really good custom arena maps already.