I would love to see a Muscle for High Stakes!
Btw, the MCO Offline mod for NFS4 was really great, but unfortunately focused on career only. Detroit Muscle mod (DM) aimed to present that time period, making use of all NFS gameplay elements (career, knockout, pursuit races...etc).
Generally i would expect from a compilated mod that everything works and looks seamless. Tracks, cars, artwork, menu-texts (generally menu style), difficulty... should fit together. Everybody can throw a bunch of cars together and call it a day.
Imo it is important that the quality of used mods does not differ too much (specifically with cars... eg. polycount, damage, sounds).
You should not use MCO conversion together with highly detailed mods if you ask me.
Another problem using cars from other authors is, the performance has to be adapted so everything works well together (handling, maximum speed...). Balance out the gameplay aspects first. Of course there should be no serial number conflicts or missing fedatas etc.
Details like a menu/HUD overhaul, showcases, new music etc are neat, but shouldnt be high priority.
Ask if you need advice/help for more specific things. I think Remko also can answer you every ambiguitys about DM mod and ideas behind it