FWIW my initial reaction is against a generic LCC forum because each of the fee-happy airlines screws its customers in idiosyncratic ways. Most RyanAir tips will not apply to Spirit and vice versa.
I'm not so sure this is totally true. There are common fees to avoid. I could envision some tips common to travel on all budget/low cost carriers such as how to minimize the weight of bags. If we have separate fora for each of them, it will potentially add a lot of fora, many with little traffic.
I would have suggested having regional budget/low cost airline fora, but a few of them (Norwegian?) are trying to expand beyond their own regions. If this continues--and if it works for some of them this time around--it could lead to confusion about which carrier belongs where, so I'm inclined at the time to favor a single forum for budget airlines and LCCs in the airlines/FF programs section of FT.