My prediction for LTC difficulty and invest/return
 

Ive done a few basic calculations and trying to figure out it its still worth mining
So today the difficulty is 364.36 if the difficulty keeps going up at 40 every 3 days in 6 months the difficulty will be 2820.00. As im mining at 1.6Mh the price at the end of 6 month for LTC/USD would have to be $8 each for me to break even getting .55 LTC per day.

If it goes up by 50 every 3 days its 3430.00 i would need the price to be $10 per LTC

Do you think LTC will be $8 in 6 month time?

So after accepting bitcoins, a site should accept also litecoins and whatevercoin?

Depends on the exchange rate to BTC or if you plan on holding the LTC in hopes it follows BTC. So far it was looking like it was going to. So if you're not making profit mining LTC I'd assume you wouldn't be able to profit from mining BTC either. Maybe mine NVC which also has an exchange of 0.02 to BTC and a very low difficulty.