A long time ago, I was involved in a project named Folkspraak to create a Germanic conlang. It was entertaining, but it did not went very far at that attempt.
Some grammatical hallmarks of Germanic languages are:
- A very simple tense system, present and past only, all other tenses are periphrastic and later acquisitions
- A past participle as third principal part of the verb
- Weak verbs ("regular" verbs) and strong verbs (looking irregular, but in fact they are regular, too, falling into several classes of strong verbs)
- Related pairs of strong and weak verbs like fall/fell where the weak verb is a causative of the strong one one (to make fall). More examples for this relation from German trinken (strong verb, "to drink")/tränken (weak verb, "to water (animals)"); springen (strong verb, "to jump, to leap")/sprengen (weak verb, "to explode; to water (lawn); to gallop"). A notorious one is erschrecken (strong verb; "to get frightened, to startle)/erschrecken (weak verb; "to startle so., to frighten")—notorious because the infinitive and some other form coincide and there is some confusion and a tendency to use sich erschrecken (weak verb) instead of the original strong verb in Modern colloquial German.
- Modal verbs that are special (preterite-present)
For nouns there is less common ground for the Germanic languages, while German retained four cases and three genders for nouns, those features are partially or completely lost in some other Germanic languages. You can do whatever you want (strive for simplicity or keep a more complicated system at your wish). Same for adjectives.