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although you have been to Thailand/Vietnam/China before there is still much more to see in SE Asia.....for example...Lao, Cambodia, Indonesia, Burma, Malaysia. As travelling in SE Asia is new for your friends a compromise would be going there because they want to but you pushing for these countries you not been to.....also in the countries you have been to there is quite possibly regions you skipped on your last trip so again push for going to these area's. If your friends are very keen to see places you been to already you could consider going solo somewhere else for a few weeks and then arrange to meet up back up again.

5 months is a decent time to explore a region/area so there are plenty of other options outside SE Asia that would be cheap to travel in.....though airfares would be more expensive if you were to consider Mexico/Central America or South America. An airfare to India would be around the same as SE Asia. Having travelled in all the above (except Central America) i would recommend all of the them. If your worried about daily spending then India would be the cheapest and in 5 months you could see a lot though July/August is monsoon season, shouldnt put you off just if you wanted beach time in Goa it would need to be planned for the end of your trip. If history/culture/ruins interest you then Mexico/C.A. would be a good option....cheap to travel in (not as cheap as india/SE Asia) and again 5 months would be a decent time to see a good bit. South America is a fantastic continent to travel in and in 5 months you could do a route like Colombia, Ecuador, Venezuala, Peru & Bolivia.....popular well travelled countries with lots to see and do and again cheap to travel in