With the advent of the MM, their really is enough information to create customized monsters, it just takes more work until the DMG comes out in November/December.
A huge number of creatures are available since last Friday. For a lot of cases, just dropping in the 5e version and adjusting #s of creatures for balance will be sufficient. Especially if the CR is 2 or above.
The basic DMG states that traits can be added/swapped without adjusting the CR. So a Veteran can become a High Elf Veteran and keep its CR. This also allows you to make a lizardfolk berserker for instance. Just give it the ability to hold its breath for 15 minutes and choose whether a +3 to AC from natural armor is better than the unarmored defense that the berserker would otherwise use.
For monsters that are not in the MM, such as an allip, there is enough information to faithfully recreate it using a 5e mindset. Choose a creatue that is as similar as possible from the standpoint of CR first and then features and traits if possible and natural vs equipment attacks if possible. Strip the traits that are not applicable and add in the traits that are applicable. The allip would get incorporeal and undead. It would also have a Wisdom version of the ability drain attack I saw on another creature that allowed the Wisdom damage to be healed on a short rest. With a few of those things added in, the allip will be perfectly serviceable in the campaign.
For classed NPCs, this will be a little more difficult if what you want is not in the NPC appendix.
I put together a CR progression of the various wizards available in the MM, starter set and HotDQ. There are only about half a dozen, and fewer if you take out the ones that have significant magic items that will skew their CR. I am not putting it up here because it would be enough information to recreate the NPCs without the MM. If you put them in a table with CR going 1 to 20 and write out the NPCs' #of hit dice, spellcaster level, and ability scores, you can then fill the blanks in with either averages or slowly increases numbers for the CRs that do not have an NPC. You can then choose a CR and see what the appropriate wizard stats would be.
This kind of setup, while it will not take long for the various NPC classes. Depending on which books you have access to, you can create these charts for fighters, rogues, druids, clerics, etc. One of the NPCs in HotDQ is even an eldritch knight. While it will take a little time to set up, it will make things a lot easier while waiting until December for the DMG.
Yes, all of these options take some work and time. But even if the DMG was already out, it would be taking time to customize creatures anyway. Just customizing a creature says that you are ready for a time commitment. I would argue that it is possible to get a good enough version of custom creatures right now.