Take these suggestions with a big grain of salt, as these are more "would-be-nice"- but I do have a few tweaks:
On the "Your Stats" page, can we get a grand total stats, as well as totals per environment? I'm just curious what my overall percentage is.
If this is possible, it would be neat if you could see other people's stats (just the summary by environment) - perhaps as a popup DIV when you mouse over their name - or a special tab where you can key in a user name and pull up their stat history?
A total number of registered players in a league would be nice to easily see, with projected prize payouts for positions finishing.
A way to "frankenstein" decks. For example, I like that I can copy a deck into another, but in the Tower's collector league I built the following decks:
RBR & Wargs
RBR & Dunlending
RBR & Sauron Swarm
RohanDalf & Wargs
RohanDalf & Dunlending
RohanDalf & Sauron Swarm
Ents & Wargs
Ents & Dunlending
Ents & Sauron Swarm
The freeps/shadows for each of these combos was extremely similar, but when I would go to build a new deck, I would have to rebuild either the freeps or shadows from scratch. I can now build the dunlending deck in my sleep since I used a similar build in the Movie Block Constructed League. I was thinking it would be neat if you could start a new deck, and choose the free peoples from one deck, the shadow from another deck, and the adventure path from a 3rd deck, or something akin to that.
It would be nice to be able to do a "deck check" on a built deck to tell you what cards are in the deck that you don't have in your "my cards" collection, so you can get a "shopping list" put together. Right now, the only way to do this is to try to start a table for "my cards" league and see the warning telling you what cards you don't own.
Also, on the Tengwar Prize selection screen, current merchant prices would be nice so you know what the best value is for getting a Tengwar card.
Finally, if you do make sharing decks available - I think a history of past leagues with a link / deckcopy of the top players would be great to keep the competitive juices flowing - if I could easily grab the winner's deck into my "deck list" so I can test it, I think it would help keep fresh ideas out there. This one is a little more tricky to privacy advocates, but I feel that once you enter a league, your decklist becomes public domain, as did Decipher, but I suppose since decks can be tweaked game to game this would be a bit more difficult to implement.
Just some random ideas on how to, in my humble opinion, improve this awesome site.