Ok so theory 1

When giving out the first prize (the custom sky fortress) the 9 cloud spirit was clearly going in order from 1st place and down the list. Given that Yerin and Lindon went first we can very safely assume that Eithan knows they decided to combine their prizes (as we can safely assume Eithan knows anything he wants to). Eithan decided to plan his fortess out as a garden with many uses not a living compound of sorts. So theory one is he plans to give (or already has) his prize to lindon and yerin to add to theirs even more.

Theory 2

Eithan plans to have lindon and yerin (as some point) return to SV and take the Wai Clan out and be under Lindons protection as an underlord. The variety of life he wanted in that garden was stated to be useful to at least refiners and forgers as i remember. So a flying fortress 3 times the size of normal others could in theory hold the wai clan.

Theory 3

Once the wai clan in back with Lindon, lindon will begin to teach his sister how to advance and possibly give her the heaven and earth purification wheel technique as he can no longer help her with a better iron body.

I base a lot of this off the fact that Eithan has be very generous to Lindon and Yerin over all as well as showing little interest in personal gain outside of his side plans for his home land. Eithan also has shown time and time again his ability to plan ahead and we get to see his plans slowly come together piece by piece.

Thoughts opinions?

EDIT:

While i dont believe lindon will still be in the tournament i think it would fit the whole series if, while yerin got the mark of the uncrowned, lindon was marked by NS himself furthering his whole "unique path of his own" story line that has been the main theme.