I think 343 will continue to make progress in Forge in subsequent games, just as Bungie made progress from Halo 3 to Reach.
343 had their hands full with assembling a brand-new team to create a brand-new AAA title, so it's completely understandable that extra things like Forge weren't exactly their top priority. Remember, Bungie already had two games under their collective belts before we even got a Forge at all, and its first iteration was a nightmare to work with compared to what we have now. Now that 343 has "figured it out" so to speak, I'd wager that we're likely to see a much more polished Forge in Halo 5.