Hi, I think the most interesting border crossing are Tamabil near Jaflong and Burimari near Patgram, depending on whether you like to spend more time in india or Bangladesh. Both border crossings are very relaxed.
If you cross in Burimari you can spend time in Meghalay and Assam before going to Siliguri. Both states are scenically very different and very nice.
A few weeks ago I entered at Burimari. So here is a suggestion based on my experience if you go the other way round:
Dhaka to Srimongol by train, cycle around there, Sylhet, Jaflong, back to Sylhet and from there to Mymensingh, tribal villages around there, to Bogra, to Dinajpur and Katanagar mandir nearby, to Rangpur, to Patgram. From there you could visit the Tin-Bigha Corridor. From Patgram you can take an autorikshaw ro Burimari, cross the borderr, take a bicycle rikshaw to Changrabandha and from there a bus to Siliguri.