Southeast Asia forum

If you want lots of waterfalls make sure to do the Bolaven Plateau motorbiking loop - so many waterfalls that you'll get sick of them. Every Tad on this map is a waterfall (http://www.travelfish.org/feature/83) and Tad Lo has three very nice spots to explore. You shouldn't have trouble finding people to take the trip with you if you just hang around Pakse a few days, and it is a nice town to hang around in.

If you extend your stay in Laos to 4 weeks it should be enough time to go overland from Huay Xia (after Chiang Mai), direct to Luang Prabang, skipping the party in Vang Vieng and probably skipping Savanakhet since you will likely want to do another motorbike loop to get to Khong Lor Cave. Each of those loops would make a good 5 day trip unto themselves, so that is the main reason I recommend adding some more time (though the Bolaven you could cut short if you just hit Tad Lo and Paksong or make a lot longer). Considering two weeks in the north and two weeks in the south that gives you still a couple of nights around Champasak and 4,000 Islands before moving on to Cambodia.

As far as overall border crossing goes, there is plenty of information on the site about that, and most places it is no problem. Remember to bring some passport photos as some countries require those for filing uselessly.

Malaysia for three weeks will be expensive, and if you cut it down to two you'll save time and money for other locations. You can easily explore KL in 3/4 nights, and then a trip to Penang or Malacca to see some of the beaches. Langkawi is a nice island with watersports and jet skis - but you'd have to fly there . . . So maybe best to stay closer to KL with Penang or Malacca and then take a flight directly to Bangkok.

I'd also keep open the idea that Singapore will seem expensive and may get old before a week is up. Though I've never been there, I'm simply not the biggest fan of Asian super cities beyond a few nights - you can get relaxing cheap cold beers a lot cheaper on the beaches of Thailand.
With four months its probably best not to plan for too many flights on your Thailand-Laos-Cambodia section as that way you can take it easy and wander as the wind takes you.