I can chime in on a few of your questions. Hopefully some of the vets with more experience than I can also give you some insight.
First, for keepers/dynasty leagues. Dynasty leagues are fairly simple and you keep everyone year to year. The only draft you have is a rookie draft. I admit that the fun of draft day is diminished some in a dynasty format, but it is made up for in pride. Knowing you drafted a player his rookie year and stuck with him for 3 years of horrible play, only to see him become a stud is very gratifying. Think of everyone who took LJ and imagine how they feel now that he has become the "arguable" top RB in the NFL ff-wise.
As for keepers, you keep a pre-determined amount of players. There are many differences in who you can keep, how many of what position, and so on. But the most common method I have done is if you keep a player, you lose your pick in the round where you had picked him. So if you picked LT in the first in 2005 and you want to keep him, you do not get a pick in the first round in 2006. In this format, you can choose to keep the max amount of players or no players so it gives each owner some freedom.
Now the best option for serious leagues with serious owners is auction style keeper leagues with no caps on the amount of keepers in my opinion. The way my favorite league works is we had a normal auction draft a few years back. A cap was put into place that year of $100. You could spend up to that point and no more, but less if you wanted to. Every year that you want to "keep" a player, you must cough up 20% more than what he cost the year prior. Also each year the "salary cap" increases by 10%. This makes it impossible to "keep" every player if you maxed out your team the prior year. You must also always hold a full roster in this format. I love it for the reason that it is the closest thing to real GM'ing we have come up with. Of course we do ammend the rules yearly based on a vote to make it even more enjoyable and even after 6 years we are still finding out ways to make it more enjoyable.
Now yes this method will also lessen the draft day enjoyment you may have now, but it is made up for by "trade talking" all year long. Contemplating trading away an LT for SJax straight up would not happen in most leagues, but in our league once you bring in the cost of the player, you realize it might be in your best interest. It adds a lot more control to your team.
But I don't want to write a book here and want more people to give you their ideas and league breakdowns as well so you can make an informed decision.
BUT, above all else, finding a good consistant group of guys to have a league with can be a challenge and you need to make sure that everyone is on board before making any changes. You wouldn't want to make some changes only to have owners leaving in a few seasons. At worst, keep the league the way it is and start up a second one with a new format and let anyone who wants in both join, then take on some new comers.
My Playoff Team - Please Comment
MSchaub, DMcNabb, JFreeman, KKolb
SJackson, THightower, CWells, JForsett, JSnelling, CJennings, QGanther
RWayne, RMoss, RMeachem, MCrabtree, DBowe