The best round 1 themes are ones where there is a lot of variability. This usually happens at the cap ranges from 80 to 100 million.
Any theme where you start with a single season team and modify it in some way (with twists and/or free agents). Or merging multiple single season teams into one. You get lots of variability with these types of themes.
Any theme where you have to go outside your comfort zone, i.e., albatross theme. For example, $75M theme where you must roster one $10+ million hitter.
How about a neutral ballpark theme? Must use WhatifSports Park. Or word it differently, i.e., you deduct $1 million from your salary cap for every point of ballpark effect. So if your park is +1, +2, 0, -3, -3, you would deduct $9 million (1+2+0+3+3) from your salary cap. This would essentially get most people playing in a neutral park, although not required. This can be applied to any theme, although a higher cap theme works better.
Any theme that prohibits or greatly reduces the use of cookies. This includes themes that limit seasons or requires minimum HR/9.
"Inside the Box' themes. We've done this before, setting up groups of players where you can take only one player from each group. It doesn't have to be 25 groups though. Maybe you group the most commonly selected players into 5 groups (you get to roster one player from group A, two from group B, three from group C, four from group D, five from Group E, and the remaining ten players from the field).
I know some don't like clones, but any kind of clone league where you get a bonus if you clone fewer players.
We've done bad defense leagues before. How about forcing everybody to have great defense. How do you build a roster to beat a team full of A+++ fielders and an A++ throwing arm behind the plate. How much can you skimp on innings? What ballpark should you play in?
I will add more thoughts later.