Also, would it make better sense to remove the roster requirments in a keeper league?
It makes sense to remove the roster requirements in ANY league.If everyone has the same number of draft picks, it's impossible to "hoard" any position unless less skilled owners are drafting less valuable players and leaving more valuable players on the board. The moment any owner feels any annoyance that someone else is "hoarding RBs" and his team suffers as a result, it should be a message to the owner that HE is at fault for improperly valuing players and he should reevaluate how he drafts. He had the same opportunity to draft RBs and he didn't, and if now his team is hurt by it, then he should learn from his mistake.
Why subsidize owners drafting poorly by creating rules so skilled owners who know what they are doing can't draft the higher valued players that are being left available and instead have to waste a roster spot on a backup TE/K/D who won't be any better than what should be available from waivers? Trying to have a league setup where skill wins out should be high amongst our goals. Roster requirements may be the rule that single most effectively strangles the options for a skilled owner to put that skill to work. It also fosters trading when owners who were unwise in how they drafted have to then turn to other teams to acquire the players they should have drafted but didn't.
As for the other effects of a flex though... generally the next 10 highest scoring players will be 8 to 10 RBs with maybe 1 or 2 WRs in there. This will increase the value of RBs which means they should be drafted even earlier and more frequently.
Personally I don't think increasing RB value of what it already is, is a good thing. My preference is to see other positions increase in value to where you don't see 2/3 of the first two rounds being all one position. Were it me, I'd rather go with a QB/RB flex and an additional WR/TE flex. The QB flex would almost always be a QB so it wouldn't increase the values of RB. Or barring that, I'd probably rather just have a WR/TE flex and leave RBs at 2 starters. Though if your league doesn't mind seeing RBs become more valuable, then the RB/WR/TE flex would be fine.
And of course, drop the roster requirements.