I'm terrible at writing intros and stuff so I'll just get straight to it:
Basically, I think this mod is amazing as is, but there is still a lot of room for new features. Here are a few ideas I came up with:
1. We already have parts for Apollo-Venus / Apollo-Mars, but what about a set of parts for an Apollo-era Mars landing? So far I've come up with a few ideas. Keep in mind everything I am about to propose was designed in RO/RSS. The MEM (Mars Excursion Module) could just be a LEM with a larger descent stage and a heatshield (yes, I did the math and it works). As for the rest of the mission hardware needed, I've come up with a plan to use Saturn V Multibody's (Saturn V MLVs [yes I know MLV stands for 'Modified Launch Vehicle', but this ties in better to Eyes Turned Skywards, which BDB already incorperates]) to put a Mars lander, a refueling depot for the lander, and the Apollo-Mars orbiter into Martian orbit.
Above you can see (left to right): A Saturn V MLV, the MEM with its de-orbit motor and its heatshield, the standalone MEM descent and ascent stages, and finally, the OPD (Orbital Propellant Depo). The OPD will also have habitation space so it can be the core module of a space station. I'll link more blueprints at the end of this post.
For reference, the Saturn V MLVs can put 45t on a Martian intercept.
First, two Saturn V MLVs put the MEM (without 30 tons of descent propellant [Aerozene50/NTO], its mass is 43 tons), and an OPD (which weighs 12 tons dry and has 30 tons of propellant) into Martian orbit (using aerobraking to capture). The MEM and OPD dock and the OPD fuels the MEM, then the MEM undocks and enters a close orbit with the OPD. Then two more Saturn V MLVs put two modified S-IVBs (added secondary propellant tanks [insulated], a heat shield, comms, radiators, docking ports, etc) into Martian orbit, each carrying 35 tons of propellant (LH2/LOX). The S-IVBs dock with the OPD and then wait.
Another Saturn V MLV puts an A-MS (Apollo-Mars spacecraft) with an added heatshield onto a Martian intercept trajectory (a fast transit on a free-return trajectory, this minimizes risk and the time the crew is in interplanetary space). The A-MS aerobrakes into Martian orbit discards it heatshield and docks with the OPD. The A-MS is then refueled (the A-MS needs 65 tons of propellant for Trans-Earth Injection) and the S-IVBs (S-IVB 1 is docked to the OPD, S-IVB 2 is docked to S-IVB 1) undock from the OPD and the MEM docks to the OPD. The crew then transfer to the MEM, undock, land on Mars, perform surface ops (probably at a prepositioned base) for 1.5 years (definitely at a prepositioned base), launch, dock with the OPD, and then transfer to the A-MS.
The A-MS then undocks and performs the Trans-Earth Injection burn (a fast transit trajectory). 150 to 300 days later, vehicle swings by the Earth. The Apollo CSM undocks, and captures into a highly elliptical orbit, from which it then enters the Earth atmosphere and splashes down in the Pacific.
I think something like this could tie in very well to BDB, and of course would require a minimal number of new parts.
2. The X-20. As far as I'm aware, there isn't a Dynasoar in BDB yet. The X-20 was an amazing concept and had a lot of really cool space station and Moon mission plans associated with it (seriously, take a look at some of the Moon mission plans using the X-20).
3. Gemini EOR (Earth orbit rendezvous) and the Saturn C-3 were really cool (and very Kerbal) mission ideas. Basically, two Saturn C-3s would put a Gemini spacecraft with a trans-Earth stage and light (like open-cockpit light) lander, and a Trans-Lunar Injection stage into Earth orbit. They then dock and head to the Moon. After capturing into lunar orbit, one astronaut head to the surface and back in the light lander. Then the Gemini spacecraft heads back to earth. In real life it was scaped due to is small scale (one person on the lunar surface for a couple of hours), and its complexity/level of risk. However, I think something like that would fit very well into KSP!
4. Moonlab / The ability to launch Skylab with fuel in it, and have living space after the fuel is depleted (you need this anyway for Apollo-Venus / Apollo-Mars). Basically, if you launched Skylab with fuel in it, it could get into lunar orbit. This was something they actually considered for the Apollo Applications Program. Unfortunately, it got scrapped. In KSP it might work quite well. To make it useful you could program a contract that says 'Conduct long-term experiments on Microgravity and exposure to deep-space radiation around the Moon Mun'.
5. Nova. Do I really need to explain this?
6. Saturn-Shuttle. Basically, by replacing the SRBs on a shuttle with a modified S-IC (which would be reusable), you could increase payload capacity and crew safety. The main addition to BDB here would be the adapter to the shuttles external tank (the shuttle stack would be mounted atop the S-IC), and the reusability of the S-IC.
7. This one isn't new parts. I'm actually not even sure if its possible. Basically, you now how KSP has part windows, like 'Filter by Function' or 'Filter by Manufacturer'? Well, what if you created a custom one for BDB, and in it had sub-folders called 'Apollo' or 'Titan' or 'Skylab' etc. It could be really useful!
I really hope you like some of these suggestions, if we're lucky we migh get some of them in BDB at some point!
More blueprints: https://imgur.com/gallery/zK4qiie