Planning a route can be very easy and you could have it done in five minutes, or you could give it more reflection.
What you should consider
In southeast Asia you have to be aware that you cannot stay for as long as you want, there are different rules for each country.
That said, if you want to extend your journey, you could do one or several Visa-run(s). It is basically a going and return to the closest country and get a new stamp on your passport. You can do that in one day or less, and you can actually book tours in travel agencies for doing so, the same way you would book for some waterfall tour.
For Thailand, you can stay on the territory without a visa for up to 30 days if you crossed the border by the air, otherwise it is 15 days by the lands
For Laos and Cambodia, you will need a visa and you can get it directly at the border for about $30. This will allow you to stay one month on the territory.
The points of interest
If you absolutely want to see or do something, then you should list them and highlight them on a map so that you can figure out the best way to minimize the transit. Your route should be coherent unless you like the buses and wasting your time.
One thing you should consider, especially if you are planning a long journey is your tiredness. Obviously, if you are traveling for a long time, you are going to be exhausted physically and mentally near the end, so you may want to go to some quiet place that is not too effort demanding.
For example, if you want to lay in hammock at a beach or on some island and you want to rest your mind, it would be wise to save these destinations of yours for the end.
Let’s make the equation a bit more complicated, but it is very important to be aware of what kind of weather you will be dealing with for a specific place at a specific date. You wouldn’t enjoy the beaches under a big rain, right ?
If you are staying in Thailand, or Laos, or Cambodia, or southern Vietnam, the temperatures shouldn’t be a major issue, but watch the monsoon though.
In northern Vietnam for instance, it can be cold at certain periods of the year. Gather all the information you need and do the math.
Let’s talk about this monsoon. Unfortunately, you cannot avoid it in this area. It is supposed to last basically between early April to late October. The rest of the time is actually a really nice period (between November to March). The temperatures are always sufficiently high but not too high at the same time and there is not much rain. It is the best season to fully enjoy southeast Asia.
What kind of traveller are you ?
The extreme backpacker
If you want to live an amazing experience, then do not ask questions, just pick a departure and a returning point. As for the rest, you will improvise. It is not a bad way to travel as a matter of fact! And it will be even more exciting without carrying a guidebook.
The big lines
It is a good way to travel. You just pick a few points of interest and roughly draw a line on the map that you would follow more or less, depending on how you feel, what you discover, and who you meet.
You can consider doing a loop or even pick a completely different returning airport but keep an eye on your watch so that you are not lacking time for seeing everything you wanted to see.
Organising every visit in every city and make the bookings in advance is a good way to be comforted before taking off.
This means, no room for the unknown, you are likely to be blocked in various situations or you wouldn’t have a choice but to leave some people you met on your way and with whom you would have stayed longer, etc.
That is THE option to ban.