I voted for a traditional 2 on 2 tag match. Any type of gimmicky, multi-team (3+) match should be for the #1 Contendership. This formula works best for the credibility and allure of the Tag Titles. The 2 on 2 match allows for a weighted, meaningful storyline (try doing that with 4 teams), and the #1 Contendership makes the titles feel more important if people are fighting for a shot at it.
Also, I'm glad you noted that everyone keeps including a Fatal Four Tag Match in their Wrestlemania Cards. I don't ever say a negative word, because I appreciate people being active on the site and sharing ideas, BUT it's very frustrating that people feel the need to include every guy on the roster in their proposed Cards. It'll say,"Hell No vs Rhodes Scholars vs The Usos vs PTPs." No! Just Hell No vs Rhodes Scholars! Haha. The Usos don't have to be on the card, leave them off. It's okay, they'll survive.
Rant nearly over... the only hodge-podge, thrown-together, match-to-get-more-names-on-the-show that really belongs at Wrestlemania is a MITB Ladder match. If you don't know what to do with 6 midcarders, put them there. It can steal the show, give young guys a chance to shine, and it's much better than a 4 v 4 This Celebrity vs That Celebrity match.