I have not made any scenarios but there are two things I would like to bring up for those that do.
1. Anytime you enter new units, make the enter time variable and random. A scenario can play out so different if your tiger gets there 10 minutes later and their fresh platoon gets there 10 minutes early.
2. This is a big one. NEVER have reinforcements arrive on a part of the map and be in instant view of any part of where the current battle would realistically be happening. If you have tanks coming onto the map at least have them enter an area protected by trees or a hill so the player at least has a chance to hear them for a turn instead of insta death. I am playing a CMBS battle right now where, without warning at all, some T72's just arrived in full view of my Stryker platoon and ruined the battle. At least one turn warning through sound or Intel would make for much better gameplay.