I have been trying to book a return airline ticket for 11 month from current date but all

airfares search engines will only allow max 10 months.. am I missing something here?

Yes .... about one month from the sound of things.

Most airlines will only allow you to book 300-330 days in advance.

I believe EVA has some one year options; Premium Economy.

Check KLM

Find the airfare you want then talk to the airline directly and tell them what you want to do. Or talk to a travel agent. The full year is possible if you need it.

Make a fake return date and then go back and revise it when you are after at the 330 return date

Most airlines will not levy a fee for changing the "outbound" flight date. At least that is what I have done with Thai and Cathay Pacific.

Eva will only let you do the return after 330 day date if you choose the fare option that allows it, which is a higher fare and will be found in the "Fare Application Period" where the fares appear

Just used KAYAK to check return flight LHR-BKK 17/6/15-17/5/16, + 3 days (11 months as required by OP)

Turned up over 550 pages of flights. Last page I looked at was no.12 before it got too boring.

Last entry on that page:

Thu 18 Jun – Wed 18 May +1 day (Trip spans 337 days)

£493

Lufthansa

Select

Lufthansa

09:30

LHR

09:45

BKK

18h 15m

1 stop (FRA)

23:40

BKK

14:40

LHR

21h 00m

1 stop (FRA)

Also checked same from CDG: BA (via LHR) £345. Originate LHR and the cheapest BA flight is £670!

GBP345 from Paris is very cheap.