G'day Allenpeng and welcome to the forum mate.
Persopnally, I think you are probably very correct in what you are attempting to do mate.
I further think that these hybrid or "crossover": type campers are he way of the future for many of us who are fast approaching the age were a true camper trailer is becoming too much to handle ie to heavy and difficult to pitch. Yet we don't feel old enough to regress to a bloody true caravan yet.
Nothing of this nature is available here that is anywhere like in the price range of the average bloke on average wages. I think this is why so many of us build our own. If you can offer a genuine crossover style camper at a more affordable price, then I say go for it.
To try to answer your questions though, here are my humble thoughts,
1. Pop-top style: will people want to spend more money on this pop-top design or just keep flat top and save cost?
I would suggest adding a pop top as an option.
2. Silde out kitchen set: do you like to move the kitchen to outside of the caravan so it can be cleaner inside and more space?
This would be a must have for me.
3. Gate position: as you can see the gate is at the rear of the caravan while some models have gate in the front? Is that important?
I would prefer the gate/door at the rear with the ISLAND BED at the front over the slide out kitchen.
4. Whole length: I need to control it no more than 6000mm because the shipping container is 6000mm or 12000mm. What length do
you prefer?
I am looking at building something smlilar to your picture (for myself) I was thinking along the lines of a body length of about 3500mm with a drawbar length of 2500mm. This would provide space for a fridge/generator box and an adequate/big tool box in front on the drawbar..
5. Colors: What colors do you prefer for the mainbody? Grey or beige or white or black? We made these four colors camper trailer.
And the drawbar is hot dipped gal - do you think we need paint the drawbar or just
I think you probably should look at offering a choice of colours. My preference would be for a military style drab olive or tan.
Good luck with your venture mate. I think you are on a winner provided you build a robust and functional trailer. Work out how strong you think it needs to be and then double that figure and you will be getting close.
Mick