What would Bangkok be like now if Thailand had undergone a successful Communist revolution? Of course, the answer to the question depends to a considerable extent on how the revolution took place, the nature of the ideology of the victorious revolutionaries (i.e. more Marxist-Leninist or Maoist), how much opposition there had been and so forth. These issues would have had a direct effect on the treatment of those who opposed the revolution: for example, if there had been a long and desperate struggle, such as in Vietnam and Cambodia, then the opposition could expect some harsh treatment – at best, thousands (maybe millions) would have fled overseas to establish resistance, government in exile and so forth from other countries (many would have suffered predation in the same way that Thai pirates attacked, raped and robbed so many Vietnamese boat refugees). At worst, large-scale re-education and labour camps would have been established and, no doubt, many would have died in their confines.
As for the city itself, it seems likely that many existing monuments and buildings would have been re-dedicated to other purposes; while new monuments to Communist leaders and victories would be found. Development of the city would have been lower overall, presumably, although public housing projects and public transportation systems might be better or at least more extensive than they are now.
What would have happened to the ethnic Chinese people in the city? Ethnic Chinese suffered under the Cambodian and Vietnamese revolutions, which initially received support from the Soviet Union. If the same pattern were repeated in Bangkok, many thousands of Sino-Thais would have found themselves persecuted and probably driven out of the country, with their capital (since Chinese are regularly accused of becoming rich at the expense of others around Asia). Russian and Russian-trained engineers would have increased the industrial estate systems, although foreign investment and ownership would presumably have been outlawed and factories would be present at the peripheries of the city as they were then. Collectivisation of agriculture would presumably have failed but not before the creation of large, central markets where goods would have been made available. Some wats may have survived but in reduced conditions.